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Book "FSS 
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COPYRIGHT DEPOSIT. 



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SPECTACLES 



AND 



EYEGLASSES 

THEIR FORMS MOUNTING AND 
PROPER ADIUSTMENT 



BY 

R. J. PHILLIPS, M. D. 

If 

OPHTHALMOLOGIST PRESBYTERIAN ORPHANAGE, LATE ADJUNCT PROFESSOR OF DIS- 
EASES OF THE EYE, PHILADELPHIA POLYCLINIC AND COLLEGE 
FOR GRADUATES IN MEDICINE, ETC. 



FOURTH EDITION, REVISED 

WITH 56 ILLUSTRATIONS 



PHILADELPHIA 

P. BLAKISTON'S SON & CO. 

1012 WALNUT STREET 
1908 



^r 









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<V 



LIBRARY of CONfifiESS) 
Two Coole« Receive 

MAR 30 1*00 

^/ >£ /f*# 

(JU89 A AXo. flu. 
^<? O /AT 
COPY 8. 



Copyright, k 



by P. Blakiston's Son & Co 



Printed by 

The Maple Press. 

York, Pa. 



PREFACE TO THE FOURTH EDITION 



New ideas in the construction of spectacles, and es- 
pecially in the construction of eyeglasses, continue to 
appear. In the present edition a description of the latest 
forms and methods of eyeglass fitting has been inserted. In 
bifocal lenses, also, there are new inventions to be de- 
scribed. 

The art of spectacle fitting has greatly improved, and 
is now better understood than when the first edition of 
this book was published. Remembering his own novitiate, 
the author's aim is still to give practical help to the novice 
in learning the principles which underlie the art and in 
choosing that form of apparatus which best serves his pur- 
pose. 

To the manufacturing opticians of his city, and partic- 
ularly to Mr. A. Reed Mclntire, the author wishes to ex- 
press his obligation for freely imparted information as 
to processes of manufacture. 

February, 1908. 



PREFACE TO THE FIRST EDITION. 



This little work is the outgrowth of the instruction on 
the subject of prescribing spectacle frames which has been 
given to successive classes at the Philadelphia Polyclinic 
and College for Graduates in Medicine. The book, like 
the teaching referred to, is intended to supplement studies 
in refraction, and to give the student that knowledge of 
the correct placing of the glasses before the eyes without 
which the most painstaking measurement of the refraction 
will frequently fail of practical results. With the populari- 
zation, as one may call it, of ophthalmology in the pro- 
fession, many physicians who prescribe glasses are com- 
pelled, by the lack of skilled opticians in their neighbor- 
hood, to themselves furnish the spectacles to the patient. 
To these, it is believed, the knowledge which I have en- 
deavored to impart in these pages will prove especially 
useful. 

Of late years much advance has been made in the art of 
making efficient, comfortable, and handsome contrivances 
for holding glasses before the eyes, and the increased use 
of prismatic and cylindrical lenses has given the fitting of 
the frames increased importance. Text-books of refraction 
remain, however, almost devoid of reference to the subject, 
the scant literature of which is scattered through opticians' 
trade publications and a few medical periodicals. Free 
application has been made to such sources, and the in- 
debtedness incurred duly acknowledged in the text. 

My thanks are due to my friend and instructor, Dr. 



VI PREFACE TO THE FIRST EDITION. 

Edward Jackson, for many valuable suggestions in writing 
this treatise, and, indeed, for directing my attention to the 
need of a book on spectacles. 

Dr. George M. Gould kindly furnished me with some 
references used in the introduction, and I am indebted to 
Messrs. Wall & Ochs, Bonschur & Holmes, and J. W. 
Queen & Co. for a number of cuts. 



CONTENTS. 



PAGE 

Introduction, i 

I. General Considerations, 9 

The Material of Frames, 9 

The Component Parts of Spectacles, 11 

The Lenses: Their Material and Manufacture, 12 

Eye Wires, Temples, and Bridges, 21 

The Different Patterns of Spectacles, 21 

Bifocal Glasses, 26 

The Varieties of Eyeglasses, 30 

Spectacles for Cosmetic Effect, 35 

II. The Principles of Spectacle Fitting, 36 

Centering and Decentering, 36 

Prismatic Effect of Decentering, 38 

Normal Lateral Centering, 41 

Normal Vertical Centering, 43 

Distance of the Glasses from the Eyes, 44 

Perpendicularity of the Plane of the Lenses to the Visual Axes, . 45 

Periscopic Glasses, 48 

III. Prescription or Fpames, 51 

The Measurements Required, 51 

Obtaining the Interpupillary Distance, 54 

Height of the Bridge, 57 

Relation of the Top of the Bridge to the Plane of the Lenses, . . 58 

Width of Base, 59 

Angle of Crest of Bridge, 59 

Prescription of Eyeglasses, 62 

IV. Inspection and Adjustment of Spectacles and Eyeglasses, . 63 
- Proving the Strength of Lenses, 63 

Phacometers, 64 

Neutralization of Spherical Lenses, 66 

vii 



Vlll CONTENTS. 

Neutralization of Cylindrical Lenses, ■ . 67 

Neutralization of Sphero-cylindrical Lenses, 68 

Locating the Optical Center, 68 

Finding the Apex of a Prism, 70 

Measuring the Strength of a Prism, 70 

Detection of Scratches, Specks, Flaws, Etc., 77 

Irregularity of the Refracting Surfaces, 77 

Adjusting Spectacle Frames, 77 

Adjustment of Eyeglasses, 83 

The Care of Spectacles, 87 



LIST OF ILLUSTRATIONS 



FIG. PAGE 

i. Position of the parts of spectacles, n 

2. Position of the parts of eyeglasses, n 

3. Sections of lenses, 15 

4. Optician's lens-grinding lathe, 18 

5. Concave toric and concave cylindrical lens, 21 

6. Frameiess bifocal spectacles, 22 

7. Forms of spectacle bridges, 23 

8. Ovals, showing the sizes of spectacle eyes, 25 

9. Forms of bifocal glasses, . . . . 26 

10. Patterns of lenticular segments, 28 

11. Extra front, 29 

12. Frameiess eyeglass, 30 

13. Coil-spring eyeglass, 31 

14. Bar-spring eyeglass, 31 

15. Rigid bridge eyeglass, 32 

16. Patterns of the offset guard, 34 

17. Spectacles with lenses decentered "in," 37 

18. Section of a normally centered lens, 38 

19. Decentered lens, showing prismatic effect, 38 

20. Profile view of the face, showing the "natural" position for the 

spectacle bridge, 43 

21. Top of bridge " out " from plane of the lenses, 44 

22. Top of bridge "in" from plane of the lenses, 44 

23. Spectacles facing directly forwards, 46 

24. Spectacles facing downwards and forwards, 46 

25. Spectacles facing inwards, 47 

26. Front and back of a convenient spectacle rule, 52 

27. Method of measuring the height of a spectacle bridge, 54 

28. Simplest method of measuring the interpupillary distance, ... 55 

29. Maddox pupil localizer, 55 

30. Method of using the Maddox pupil localizer, 55 

31. Meyrowitz pupillometer, 56 

32. A common form of pupillometer, ; . 57 

ix 



X LIST OF ILLUSTRATIONS. 

33. Method of measuring the distance of the bridge "out," 58 

34. Method of measuring the distance of the bridge "in," 59 

35. Implement for measuring angle of crest of bridge, 61 

36. Use of a crest measure, 61 

37. Mr. Brayton's lens measure, 65 

38. Apparent displacement of lines caused by rotating a cylinder . . 67 

39. Method of finding the axis of a cylindrical lens, 67 

40. Ready method of locating the optical center of a lens, 69 

41. Mode of marking the apex of a prism, 70 

42. A prism improperly held, 70 

43. Implement for measuring the refracting angle of prisms, .... 74 

44. Method of determining the deviating angle of prisms, 75 

45. Rotation of a lens -within its eye wire, 77 

46. Bend at the junction of the eye wire and bridge, . . , 78 

47. Showing the planes of the lenses crossing each other, 78 

48. Bend of the bridge, 79 

49. Inequality of corresponding angles of the bridge, 79 

50. Angles on one side of the bridge too small; on the other too large, . 79 

51. Proper fitting of hook temples, 81 

52. A common but incorrect shape of the nose-pieces of eyeglasses, . . 83 

53. Eyeglasses with nose-pieces of correct shape, 84 

54. Diagram showing inclination of nose-pieces, 85 

55. Diagram showing inclination of nose-pieces, 85 

56. The points at which eyeglasses are to be adjusted, » 86 



SPECTACLES AND EYEGLASSES. 



INTRODUCTION. 

At what time man invented lenses and discovered the aid 
which they are capable of lending to vision is a matter 
beyond our knowledge. It is tolerably certain that they 
were known to civilizations earlier than ours. Though it 
might be difficult to prove that spectacles were known to 
the ancients, the evidence in relation to their acquaintance 
with the essential element of spectacles, the lens, is reason- 
ably convincing. This evidence was, for the most part, 
discovered by Sir Austen Henry Layard among the ruins of 
old Nineveh, and is of the most interesting character. 
Among the articles which he unearthed was a specimen of 
transparent glass (a small vase or bowl) with a cuneiform 
inscription fixing its date quite accurately to the latter 
part of the seventh century B. C. ("Discoveries Among 
the Ruins of Nineveh and Babylon, etc.," by Austen H. 
Layard, New York, 1853, p. 196.) This is the most an- 
cient known specimen of transparent glass, though Egypt 
furnishes it of a date only a century later, and opaque or 
colored glass was manufactured at a much earlier period, 
some specimens of the fifteenth century B. C. still endur- 
ing. However, the ancient nations were not compelled 
to wait for transparent glass in order to invent lenses, 
as they had in rock crystal a material admirably adapted 
to that purpose, and Layard was so fortunate as to 
discover such a lens in Nineveh. (Ibid., p. 197.) Sir 
David Brewster, who examined this lens, described it 



2 SPECTACLES AND EYEGLASSES. 

as being plano-convex, of a diameter of one and a half 
inches, and capable of forming a tolerably distinct focus 
at a distance of four and a half inches from the plane 
side. It is interesting to note further in regard to this, 
the oldest lens in existence, that it is fairly well polished, 
though somewhat uneven from the mode in which it 
was ground, which Brewster concludes was not upon a 
spherical surface, but by means of a lapidary's wheel, or 
some method equally rude. Another evidence of the use 
of lenses has come down to us from antiquity. Upon 
record cylinders of old Nineveh, and on engraved gems and 
stones of Babylon, Egypt, and other sources which long 
antedate the Christian era. are characters and lines of such 
delicacy and minuteness as to be undecipherable without 
the aid of a magnifying lens. Taking these facts in con- 
junction, the statement that some of the properties of lenses 
were known to and utilized by the ancients, the old record 
writers of Assyria, for instance, may be regarded as almost 
as well demonstrated as though it were made of a modern 
engraver, and we were to step into his workshop and find 
his magnifying loup lying beside his work. 

The testimony as to their use by the Romans during 
their supremacy is of a less conclusive character. The 
statement frequently made that the Emperor Nero used a 
concave jewel to assist his sight rests upon some obscure 
sentences in Pliny. That author says: "Nero could see 
nothing distinctly without winking and having it brought 
close to his eyes." (Bk. n, Chap. 54, Riley's Trans.) In 
another place, speaking of the emerald, smaragdus, he 
says: "In form these are mostly concave, so as to re- 
unite the rays of light and the powers of vision. * - * * 
When the surface of the smaragdus is flat, it reflects the 
image of objects in the same manner as a mirror. The 
Emperor Nero used to view the combats of gladiators upon 
(with, or by means of) a smaragdus." (Bk. 37, Chap. 17.) 



INTRODUCTION. 3 

The mention of the reflecting properties of the emerald 
immediately before the statement of Nero's use of it, with 
the alternative renderings of the Latin ablative, smaragdo, 
make the supposition that Nero used the emerald as an 
eyeglass uncertain, though in view of his clearly described 
nearsightedness, the conjecture is probable enough. 

Lenses appear to have been unknown in Europe during 
the first twelve hundred years of the Christian era, though 
the Saracen Alhazen, who died in Cairo in 1038, has left 
books showing his acquaintance with them. These books 
were brought to Europe at a very early period, and the 
manuscripts yet exist, some in the Bodleian library, and 
another portion in that of the University of Leyden. 
They treat with remarkable clarity and accuracy of the 
laws of reflection and refraction, including reflection 
and refraction by surfaces convex, concave and cylin- 
drical. Some of their diagrams showing the course of 
light rays are in use in our text-books at this day. In 
spite of some errors, they exhibit, also, a good knowledge 
of the anatomy and physiology of the eye. It was prob- 
ably from these works that the early writers obtained 
their first hints of the science of optics, on the revival of 
learning in the fourteenth and fifteenth centuries. In 1572 
a Latin translation of Alhazen's treatise on optics was 
published at Basle, and in 1600 Johann Kepler, the as- 
tronomer, wrote the first European work on the subject. 
It is worthy of note that Alhazen was born at Bassora, at 
the head of the Persian Gulf and less than five hundred 
miles from the spot where, sixteen hundred years before, 
had stood the palace of the Assyrian kings in the ruins of 
which Sir Henry Layard found the lens of crystal. It 
might, perhaps, be plausibly maintained that in the coun- 
tries about the Tigris some knowledge of optics, and of 
convex lenses, has persisted without eclipse from the most 
remote ages. 



4 SPECTACLES AND EYEGLASSES. 

We are told in a general way that the Chinese have for 
ages employed spectacles for the relief of defective eye- 
sight. This is, perhaps, to be regarded as only another 
instance of the exercise of that claim to priority which the 
Chinese are known to extend over every good and perfect 
gift. It is known, however, that the countries about the 
Mediterranean, Greece, Rome and Palestine, each in its day 
of grandeur, had some intercourse and trade with China. 
Europe undoubtedly received the science of optics in a 
fairly advanced state from the Arabs. The Chinese may 
very well have obtained their knowledge from the same 
source. So far as is shown by the evidence, the ancient 
Assyrian was the first to make and use a lens, and the 
Arab the first to write a scientific treatise on optics. 

The earliest European reference to our subject occurs in 
the writings of Roger Bacon, who died in 1292, and to 
whom the invention of the instrument he describes is 
sometimes accredited. Bacon's glass was apparently a 
large plano-convex lens, probably what we now call a 
reading glass, intended to be held in the hand, and of it he 
says: "This instrument is useful to old men and to those 
that have weak eyes; for they may see the smallest letters 
sufficiently magnified." Spectacles proper — that is, glasses 
mounted so as to retain themselves upon the face — appear 
to have been invented in Florence somewhere between 
1280 and the close of the thirteenth century. Dr. Samuel 
Johnson is said to have expressed surprise that the inventor 
of such useful articles has found no biographer. Doubtless 
among the thousands for whom the discovery has kept 
open the sources of knowledge there would be found one 
to pay this tribute to the fame of his benefactor were the 
identity of the latter a matter of certainty. But, unfortu- 
nately, our evidence on the point is of the most fragmen- 
tary character. The tomb of Salvinus Armatus, a Floren- 
tine nobleman who died in 1317, is said to bear an inscrip- 



INTRODUCTION. 5 

tion to the effect that he was the inventor. If epitaphs 
enjoyed a less equivocal reputation for truthfulness he 
would doubtless be held in grateful remembrance as the 
man who has lengthened youth by postponing old age; 
and, like Joshua, kept back the night until the day's work 
was done. 

Whoever the inventor, Alessandro di Spina, a monk of 
Florence who died in 1313, is generally accredited with 
having made public the use of spectacles, and by several 
Florentine writers of that time we find them mentioned and 
recommended. Pissazzo, in a manuscript written in 1299, 
says: "I find myself so pressed by age that I can neither 
read nor write without those glasses they call spectacles, 
lately invented, to the great advantage of poor old men 
when their sight grows weak." Friar Jordan, of Pisa, in 
1305 says that "it is not twenty years since the art of mak- 
ing spectacles was found out, and is, indeed, one of the best 
and most necessary inventions in the world." 

An early mention of spectacles, or, in the language of 
that time, " a spectacle," occurs in " The Canterbury Tales," 
where Chaucer makes the Wife of Bath use the metaphor: — 

Povert (poverty) full often when a man is lowe, 
Makith him his God and eek himself to knowe. 
Povert a spectacle is, as thinkith me. 
Through which he may his verray (true) frendes se. 

There is in existence in the church of Ogni Santi, Florence, 
an old fresco by Domenico Ghirlandajo, representing St. 
Jerome, and dated 1480. The Saint is portrayed seated 
at a desk, apparently deep in the composition of one of 
the blasts against the Heretics for which he was famous. 
Upon a peg at the side of the desk, together with the ink- 
horn and a pair of scissors, hangs a small handleless pince- 
nez.^ The glasses are round and framed in dark bone, and 
in the bridge, also of bone, is a hinge. Though the artist 
seems to have been little impressed by the fact that St. 



6 SPECTACLES AND EYEGLASSES. 

Jerome died in the year 420, nearly nine centuries before 
spectacles were invented, the mounting and material repre- 
sented in these early spectacles are worthy of note as show- 
ing their form in Ghirlandajo's time, and probably that in 
which they originated. 

In the early references to spectacles it is the convex lens 
for the use of the presbyopic which is mentioned. It 
must have been early discovered that there is a more or 
less close relation between the age of the wearer and the 
strength of the convex glass required, and the baneful 
theory was soon developed that this relation is constant, 
and that it would be ruinous to use a lens "too old for the 
eyes," a superstition from which the public is even yet not 
fully emancipated. We find it rampant in Pepys' time, 
preventing his oculist, Dr. Turberville,* from giving that 
gentleman a proper correction for his accommodative 
asthenopia, of which the diary gives an accurate picture', 
and losing to the world many a priceless page. Pepys 
says (June 30, 1668): "My eyes bad, but not worse, only 
weary with working. * * * I am come that I am not 
able to read out a small letter, and yet my sight good, for 
the little while I can read, as ever it was, I think." But 
Dr. Turberville warns him against glasses too old for him, 
and so the diary is closed, and Pepys in a last pathetic 
entry resigns himself to coming blindness; and yet the 
convex lenses were at his hand, ready to dissipate the mists 
before him and enable him to "gaze upon a renovated 
world." 

Improvement in spectacles appears to have been slow. 
The world waited more than two centuries after Kepler 
for another signal advance. Sir David Brewster is said to 
have discovered his own astigmatism; that is, he discovered 

*Daubigny Turberville; created M.D. at Oxford in 1660. He practiced 
with great reputation as an oculist in London. His monument yet remains 
in Salisbury Cathedral, where he was buried. 



INTRODUCTION. 7 

that vertical and horizontal lines were not equally well seen 
by him at like distances, but the phenomenon was not ex- 
plained and the observation faded from view. It remained 
for George Airy, the astronomer, to rediscover astigmatism, 
which he did about 1827, to determine that the curvature 
of the cornea was greater in one diameter than in another 
at right angles to the first, and to apply the cylindrical lens 
to the correction of the condition. Mr. Airy's right eye 
was myopic, while in the left he had compound myopic 
astigmatism. By a careful comparison of the appearance 
of objects when viewed with each eye singly, and a study 
of the effect of concave lenses held before the left eye upon 
lines crossing each other at right angles, he was able to 
conclude that the refraction of that eye differed in different 
planes. Mr. Fuller, an optician of Ipswich, made, under 
Airy's direction, a concave sphero-cylindrical lens w T hich 
satisfactorily corrected his refractive error. Thus was the 
last great discovery in spectacles accomplished — a bit of 
work for completeness leaving nothing to be desired, and 
of not sufficiently acknowledged importance to humanity. 
Benjamin Franklin invented bifocal spectacles. Since 
this statement is supposed by many to rest on tradition 
only, it may be of interest to quote a portion of a letter of 
Franklin's which bears upon the point. The letter is ad- 
dressed to George Whately, of London, and is dated 
Passy, 23d May, 1785. In it Dr. Franklin says: "By Mr. 
Dolland's saying that my double spectacles can only serve 
particular eyes, I doubt he has not been rightly informed 
of their construction. I imagine it will be found pretty 
generally true that the same convexity of glass through 
which a man sees clearest and best at the distance proper 
for reading, is not the best for greater distances. I there- 
fore had formerly two pairs of spectacles, which. I shifted 
occasionally, as in traveling I sometimes read, and often 
wanted to regard the prospects. Finding this change 



8 SPECTACLES AND EYEGLASSES. 

troublesome and not always sufficiently ready, I had the 
glasses cut and half of each kind associated in the same cir- 
cle. By this means, as I wear my spectacles constantly, I 
have only to move my eyes up or down, as I want to see 
distinctly far or near, the proper glasses being always ready. 
This I find more particularly convenient since my being in 
France. * * *" ("The Complete Works of Benjamin 
Franklin." Ed. by John Bigelow, New York, 1888.) 

We may infer from the context that the invention took 
place before Franklin went to France, which was in the lat- 
ter part of 1776. As he was born in 1706, the necessity 
for a double glass would first arise about 1750, and the in- 
vention therefore took place somewhere between this date 
and that of the journey to France. 

The frames in which spectacles were mounted continued 
to be very clumsy affairs until the beginning of this century, 
when light metal frames were introduced in place of the 
earlier devices of bone, horn, or shell. Their later evolu- 
tion has generally been along the lines of improved mechan- 
ical construction and increased lightness and beauty. It 
would be difficult to mention an article which plays a more 
important part in modern life than do spectacles, or one 
which plays its part more acceptably. It is scarcely possi- 
ble to estimate them at their true worth, or to imagine our 
condition without them. Deprived of their aid, most men 
would be too old for work at fifty, and purblind at sixty. 
For us all, as an old writer quaintly observes, "they keep 
the curtain from falling until the play has come to an end." 



I. GENERAL CONSIDERATIONS. 

By far the most generally useful method of placing 
glasses before the eyes is by spectacle frames, though the 
eyeglass, or pince-nez, has advantages in some cases, from 
the facility and quickness with which it may be placed in po- 
sition or removed. The superiority of eyeglasses in appear- 
ance and becomingness is another point not unworthy of 
consideration, as the glasses will surely be more constantly 
worn if they are becoming than if they are not so. More- 
over, the patient is justly entitled to the correction of his 
refractive error with as little injury to his appearance as 
possible. The disadvantages of eyeglasses are, that for 
constant wear they are seldom so comfortable as spec- 
tacles; that on some faces it is nearly impossible to keep 
them in place; while, where the contained glass is cylin 
drical or prismatic, the rotary displacement which it is pos- 
sible for the glass to take is a serious and sometimes fatal 
objection to their adoption. 

Lorgnettes and single eyeglasses, or quizzing glasses, as 
they are called, are little more than playthings; though 
sometimes, as in aphakia, or high myopia, a strong convex 
or concave lens in one of these forms is of use when the 
spectacles constantly worn do not give the vision which 
may occasionally be required. 

The Material of Spectacle Frames is usually gold, silver, 
steel, brass gold plated, or alloys containing nickel and tin. 
Horn, tortoise shell and celluloid find, or have found, a 
limited applicability in this connection. 

Gold, of from 10 to 14 karat, is, by far, the best material 
for frames. Finer than this it is too flexible, while if less 
pure it may blacken the skin. In the end, such frames 

9 



IO SPECTACLES AND EYEGLASSES. 

are cheaper than steel, as, owing to the liability of the 
latter metal to rust when in contact with the moist skin, 
the gold will outlast it many times over. In eyeglasses, 
however, the parts are heavier, and the metal is not in 
contact with the skin; so that there is not the same liability 
to rust. The gold frames furnished by opticians in this 
country usually have a stamp mark on the inner side of the 
right temple, near the hinge, which denotes the fineness 
of the gold: thus 8 karat is marked +; 10 karat, 6; 
12 karat, *; while 14 karat, or finer, is marked 14k, etc. 

Silver and brass are very poor material for frames, 
being soft, flexible and entirely lacking in elasticity. 
They are only useful for workmen's protective goggles 
or some such purpose where very heavy frames are allow- 
able. The various alloys of nickel and tin, sold under 
trade names, have the faults of silver and brass in less 
degree. Some of them make fairly satisfactory frames for 
eyeglasses. For spectacles they are scarcely worth con- 
sidering. 

Gold is, therefore, not only the best but the only good 
material for frames. A cheap frame having a certain 
amount of rigidity and elasticity, and free from liability 
to rust, is still a thing to be desired. Aluminum has been 
regarded as promising in this connection, but except light- 
ness it has scarcely a quality to recommend it. It is 
soft, flexible and inelastic, and is readily corroded by the 
perspiration if the latter happen to be alkaline, which it 
frequently is. Moreover, aluminum cannot be soldered. 
In itself, therefore, it is unlikely ever to be available for 
this purpose. Some of its alloys, however, have interesting 
properties. That composed of ten parts of aluminum and 
ninety parts of copper some authorities assert to be the 
most rigid metal known. It is a red gold color, does not 
tarnish readily, is lighter than brass or gold, and can be 
soldered. It is possible that in some such alloy will be 



GENERAL CONSIDERATIONS. 



I I 



found a material having the valuable properties of gold for 
this purpose and without the latter's cost. 

The Component Parts of Spectacles. — A pair of spec- 



FlG. I. 




tacles is made up of fifteen or seventeen pieces, 
positions are shown in Fig. i. They are: two lense 
eye wires, four end pieces, two screws, two pins, or d 



Ha 

whose 
s, two 
owels, 



12 SPECTACLES AND EYEGLASSES 

two temples, and one bridge. Sometimes the rings upon 
the temples, through which the dowels pass, are formed as 
separate pieces. Fig. 2 shows the name and position of each 
part of an eyeglass. A glance at the more important of the 
many interesting processes required in making these different 
parts will contribute to an understanding of the subject. 

The Lenses. — The word lens is the Latin name of the 
lentil, a small bean. The resemblance in shape caused the 
name to be applied to the optical implement. Spectacle 
lenses are usually made of glass; sometimes of rock crystal 
(crystallized quartz). The latter substance has a slightly 
higher index of refraction, so that a lens of a given strength 
may be somewhat lighter when made of it than when made 
of glass. The notion is common that these "pebbles," 
as they are called, possess a peculiar virtue in strengthen- 
ing the eyes or in some other direction. I suppose the 
idea is that, being the product of Xature"s laboratory, 
they are necessarily superior. The advantage which 
they may have of being slightly lighter and harder than 
lenses of glass is more than counterbalanced by their higher 
cost, and by the fact that the index of refraction of rock 
crystal is not very constant. 

Of the different kinds of glass, that known as crown 
glass is preferred, on account of the superior brilliancy 
which it possesses. It differs from ordinary sheet glass 
only in the method of blowing. At one point in the proc- 
ess, the mass of glass on the blowpipe assumes the shape 
of a crown; hence the name. Although glass is theoretic- 
ally a definite chemical compound, the different methods 
of handling make a considerable difference in the product 
of different makers. It consists chemically of silicic acid 
united with some two of the metallic bases : sodium, potas- 
sium, calcium, magnesium, aluminum, iron, and lead, but 
owing to impurities in the glassmaker's raw materials, traces 
of several more of these bases are generally present. The 



GENERAL CONSIDERATIONS. 1 3 

bases calcium and sodium are those used for ordinary sheet 
and crown glass; iron, always present as an impurity, giv- 
ing the product its greenish tinge. To lessen this tint, ar- 
senic is employed as a bleaching agent. Peroxid of man- 
ganese is sometimes used for the same purpose, and it is a 
slight excess of this agent which gives to certain samples of 
glass their pinkish tint. The transparency of such glass is 
thought to be less durable than that having the greenish 
color. For glass for spectacle lenses of the best grade our 
grinders are still dependent on Europe, no source of supply 
of a first-class and uniform product having as yet appeared 
here. 

The index of refraction of the varieties of glass used in 
spectacle making is as follows: 

Crown glass .' 1.5 

Flint glass. " 1.57 

Ordinary sheet glass 1.53 

Rock crystal 1.56 

The two broad, polished surfaces of a lens are called its 
refracting surfaces, since it is at these surfaces that the rays 
of light are refracted when the lens is in use. On the shape 
of these surfaces, and their position relative to each other, 
depend all the powers and properties of a lens. Each of 
these surfaces may be either plane, spherical, or cylindrical. 
A spherical surface is such a one as, continued in all direc- 
tions, would form a sphere, and which is, therefore, a seg- 
ment of a sphere. Similarly, a cylindrical surface is the 
segment of a cylinder. Spherical and cylindrical surfaces 
may be either convex or concave. A single surface of a 
lens may be, therefore, either 

Plane, 

Convex spherical, 

Concave spherical, 

Convex cylindrical, 

Concave cylindrical. 



14 SPECTACLES AND EYEGLASSES. 

Since every lens has two refracting surfaces, the list of 
lenses which it is possible for the lens maker to produce 
by combinations of these five primary surfaces is as fol- 
lows : — 

i. Prismatic. 

2. Plano-convex spherical. 

3. Plano-concave spherical. 

4. Plano-convex cylindrical. 

5. Plano-concave cylindrical. 

6. Biconvex spherical. 

7. Biconcave spherical. 

8. Concavo-convex (two varieties) : 

(a) Radius of curvature of convex surface greater than 
that of concave. (Converging meniscus.) 

(b) Radius of curvature of convex surface less than 
that of concave. (Dispersing meniscus.) 

9. Sphero-cylindrical (four varieties) : 

(a) Convex sphere combined with convex cylinder. 

(b) Convex sphere combined with concave cylinder. 

(c) Concave sphere combined with concave cylinder. 

(d) Concave sphere combined with convex cylinder. 

10. Biconvex cylindrical, axes coincident. 

11. Biconvex cylindrical, axes crossed. 

12. Biconcave cylindrical, axes coincident. 

13. Biconcave cylindrical, axes crossed. 

14. Concavo-convex cylindrical, axes coincident. 

15. Concavo-convex cylindrical, axes crossed. 

Sections of lenses are shown in Fig. 3, each section illus- 
trating two or more lenses, accordingly as we regard the 
curved lines as sections of spheres, or cylinders, and the 
straight lines as planes, or as sections of cylinders in the 
direction of their axes. 

Lastly, the prism may be introduced as an element into 
each of these lenses. Thus we have quite a long list of the 
possible forms of the lens, and that without considering the 



GENERAL CONSIDERATIONS. 



15 



"toric" surface, which will be spoken of later. Of these 
lenses only the nine first mentioned, and the combination of 
some of them with the prism, are in practical use. The 
others, the bicylindrical lenses, besides being difficult of 
manufacture, have each its optical equivalent in some sim- 
pler form of lens, either piano-cylindrical or sphero-cylin- 
drical. They are only mentioned now because their use 
has been advocated by a few writers in the past. 

Difficulties in grinding, and the near equivalence of cer- 
tain of the lenses mentioned among the first nine of our 
list, render the use of some of these lenses quite rare. It 
is, for instance, more difficult to grind a perfect plano- 

Fig. 3. 




spherical lens than it is to grind a bispherical, and as in 
weak lenses, such as are used in spectacles, the action of 
every piano-spherical lens can be nearly exactly duplicated 
by some bispherical one, we seldom find piano-spherical 
lenses in use. Among sphero-cylindrical lenses also it is 
usual to consider certain combinations as equivalent to each 
other. For example, a convex spherical combined with a 
convex cylindrical, as equivalent to some stronger convex 
spherical combined with a concave cylinder. These lenses 
are only strictly equivalent, however, for a small area near 
their optical centers. When their influence on the field of 
vision is taken into account, they can no longer be consid- 



i6 



SPECTACLES AND EYEGLASSES. 



ered identical, as we shall see in considering periscopic 
lenses. 

In Fig. 3, the first three lenses shown act as convergers 
of rays, and are all considered as convex, or "plus" lenses, 



TABLE I. 



OLD SYSTEM. 


NEW SYSTEM. 


I. 


11. 


III. 


IV. 


V. 


VI. 


VII. 


VIII. 


No. 


Focal 


Focal 




No. 


Focal 


Focal 


No. 


of the 


Distance 


Distance 




of the 


Distance 


Distance 


corres- 


Lens, 


in 


in 


Equiva- 


Lens, 


in 


in 


ponding 


Old 


English 


Milli- 


lent in 


New 


Milli- 


English 


of the Old 


System 


inches. 


meters. 


Diopters. 


System 


meters. 


inches. 


System. 


12 


67.9 


1724 


0.58 


0.25 


4000 


157.48 


166.94 


60 


56.6 


1437 


0.695 


0.5 


2000 


78.74 


83.46 


48 


45-3 


1 150 


0.87 


0.75 


1333 


52.5 


55-63 


42 


39-6 


1005 


0.90 


1 


1000 


39-37 


4L73 


36 


34 


863 


1. 16 


1.25 


800 


3i-5 


33.39 


30 


28.3 


7i8 


1-39 


1.5 


666 


26.22 


27-79 


24 


22.6 


574 


1-74 


1.75 


57i 


22.48 


23.83 


20 


l8.8 


477 


2.09 


2 


500 


19.69 


20.87 


18 


1/ 


431 


2.31 


2.25 


444 


17-48 


18-53 


16 


IS 


381 


2.6 


2.5 


400 


15-75 


16.69 


15 


14. 1 


358 


2-79 


3 


333 


13-17 


13.9 


14 


13-2 


335 


2.98 


3-5 


286 


11.26 


11.94 


13 


12.2 


312 


3-20 


4 


250 


9.84 


10.43 


12 


ir.2 


287 


3.48 


4-5 


222 


8.74 


9.26 


11 


10.3 


261 


3.82 


5 


200 


7-87 


8-35 


10 


9.4 


239 


4.18 


5-5 


182 


7.16 


7-6 


9 


8.5 


216 


4.63 


6 


166 


6.54 


6.93 


8 


7-5 


1 go 


5.25 


7 


143 


5-63 


5-97 


7 


6.6 


167 


5.96 


8 


125 


4-92 


5-22 


6V 2 


6.13 


155 


6.42 


9 


in 


4-37 


4.63 


6 


5-6 


142 


7.0 


10 


100 


3-94 


4-17 


s'A 


5-2 


132 


7-57 


11 


9i 


3-58 


3-8 


5 


4-7 


119 


8.4 


12 


S3 


3-27 


3.46 


aVt. 


4.2 


106 


9.4 


13 


77 


3-03 


3-21 


4 


3.8 


96 


10.4 


14 


7i 


2.8 


2.96 


3% 


3.3 


84 


". 9 


15 


67 


2.64 


2.8 


3% 


3.1 


79 


12.7 


16 


62 


2.44 


2.59 


3 


2.8 


71 


14.0 


17 


59 


2.32 


2.46 


2% 


2.6 


66 


15. 1 


18 


55 


2.17 


2.29 


2% 


2.36 


60 


17.7 


20 


50 


1-97 


2 09 


2% 


2.1 


53 


18.7 










2 


1.88 


48 


20.94 











being designated by the sign +, or sometimes by ex. 
The remaining lenses in the figure act as dispersers of 
the rays and are known as "minus," or concave lenses, 
and receive the sisrn — , or sometimes cc. For the terms, 



GENERAL CONSIDERATIONS. 1 7 

spherical lens, cylindrical lens, prismatic lens, sphero-cylin- 
drical lens, etc., the words sphere, cylinder, prism, sphero- 
cylinder, etc., are frequently employed and are unobjection- 
able. Finally, the sign O is used for "combined with" in 
the formula of a combination lens, as + 4. sphere O -f 2. 
cylinder. 

The new system of numbering lenses, the dioptric system, 
has so entirely fulfilled the requirements of the users of 
lenses, and has so simplified and facilitated our every-day 
work and calculations, that the old or inch system of 
numbering is rapidly becoming of historical interest only. 
As its use, however, still survives in certain quarters, 
and lenses are frequently met with which are marked 
by this system, a table showing the equivalence of the ordi- 
nary lenses of the test case in the two systems is shown on 
page 16. It is calculated for an index of refraction of 1.53. 

The simple apparatus used for grinding a single spherical 
lens is shown in Fig. 4. The disk of glass of which a lens 
is to be made is fastened, by means of pitch, to a small, 
cubical block of iron having a pit in the surface opposite 
that to which the glass is fastened. Into this pit fits a 
pin upon a lever, which is in the hand of the workman. 
When the free surface of the glass is applied to the surface 
of the "tool" to whose form it is to be ground, it, together 
with the block of iron, turns upon the pin. The universal 
joint at the end of the lever permits lateral and vertical 
movements, so that the workman is able to carry the glass 
freely over all portions of the tool. 

The tool which gives the shape to the surface of the 
glass is made of steel; and for spherical glasses is in the 
form of a disk, with its surfaces looking upward and down- 
ward, and revolving about a vertical axis, like a potter's 
wheel. The upper surface of this disk is convex for grind- 
ing concave glasses, or concave for grinding convex glasses. 
Of course, each strength of lens requires a separate tool 



SPECTACLES AND EYEGLASSES. 



having the requisite convexity or concavity of surface. 
The abrading material placed upon the surface of the tool 
is wet, powdered emery of successively finer and finer grades 



Fig. 4. 




Optician's Lathe for Grinding Spherical Lexses. 



until the desired amount of glass has been ground away. 
When this process is complete, the surface of the glass has 
the desired spherical curvature, but it is rough: that is, it is 



GENERAL CONSIDERATIONS. 1 9 

"ground glass." To polish it, a piece of wet broadcloth 
or felt is smoothly applied to the surface of the tool upon 
which the glass was ground, conforming, of course, to that 
surface. The cloth, being sprinkled with wet "rouge" (a 
carefully calcined sulphate of iron), gives the glass held 
against it a beautiful polish without altering its spherical 
curvature. The same processes must now be gone over 
with the other surface of the lens, after which it is cleaned 
and cut to a shape suitable for its future mounting. 

This is done by means of an implement called a lens- 
cutter, in which the lens rests on a leather cushion and is 
held firmly in position by a rubber-tipped arm, while a 
diamond-tipped glass-cutter, guided by a pattern, traces 
the oval or other desired outline upon the glass. The 
superfluous glass is removed piecemeal by means of pincers, 
and the lens passes to the next process, which is the smooth- 
ing and, if necessary, beveling of the edges. This is done 
by hand upon large Scotch grindstones. If the lens is 
to be mounted in a round eye wire, its edge must be grooved 
by means of a file, while a skeleton frame will require the 
drilling of the glass, which may be done by hand with a 
steel drill or by a special machine. 

In grinding a cylindrical lens the surface of the tool is, 
of course, a portion of the surface of a cylinder, and the 
glass is ground by a to-and-fro motion. It is evident that 
the axis of the cylinder in the future spectacle need not be 
taken into account in grinding, but only in the process of 
cutting to shape for mounting. 

When the lenses are of high power it is of advantage 
that they be made in the form of a meniscus, giving what 
are known as periscopic glasses. For instance, if a +4. 
diopter lens is required, the anterior surface is ground to a 
+ 6. D. and the posterior surfaces to a — 2. D. It is just 
as advantageous to a cylindrical or sphero-cylindrical glass 
to be periscopic as it is to a spherical, but under present 



20 SPECTACLES AND EYEGLASSES. 

methods of grinding it is manifestly impossible to give 
them this form, as the cylinder is ground on one side, and 
the other ground to a plane or sphere, as the case may be. 
Glasses which overcome the difficulty have, however, been 
made, and were described by Dr. George C. Harlan at the 
meeting of the American Ophthalmological Society in 
1885, and again in 1889. From the latter communication 
I quote the following description of the glass : 

"The lens to which I wish again to call the attention of 
the Society consists of crossed cylinders ground on one 
surface of the glass, the other being left for any desired 
spherical curve. In this way a meniscus may be produced. 
Here, for instance, is a combination lens giving the effect 
of + 4- O +2. Cyl. To produce this effect crossed cylin- 
ders of + 4. and + 6. are required, supposing the other 
surface of the glass to be left plain. If we wish to give the 
periscopic form to this glass, it can be done by making the 
cylinders 6 and 8, and grinding a — 2. sphere on the other 
side. If a simple cylinder is needed, the spherical curve 
must equal that of the weaker cylinder. 

"I learn from a recent publication by Dr. George J. 
Bull, of Paris, entitled 'Lunettes et Pince-nez,' that glasses 
similar to these have been made with more or less success 
before, but have never come into general use. Dr. Bull 
describes them under the name of 'verves toriques.' The 
tore (Latin torus) is the surface engendered by a circle 
which turns about an axis situated on the plane of the 
circle. A familiar example of the torus is the circular con- 
vex molding at the base of an architectural column. A 
glass ground upon a wheel having this form will present 
two cylindrical curves at right angles to each other, one 
depending on the radius of the wheel, and the other on the 
radius of the convexity of its rim. It would seem that 
'toric lenses' is the proper designation of these glasses." 
Fig. 5, A, represents a concave toric lens. In the same 



GENERAL CONSIDERATIONS. 



21 



figure, b is a concave cylindrical lens, introduced for the 
sake of comparison. 

Those who have used these glasses consider them much 
more satisfactory than glasses made by the common 
method, and they should be borne in mind when prescrib- 
ing for high astigmatism in patients who use their eyes a 
great deal for work requiring accuracy. 

Eye Wires, Temples, and Bridges. — Eye wires are made 
by wrapping the untempered wire, in the form of a spiral, 
closely about^a flattened metal cylinder. Being tempered 



Fig. 5. 





while in this position, the loops of the spiral will retain the 
shape given them. A single cut down the side of the 
cylinder converts each loop into a separate oval ring. End 
pieces and straight temples are stamped from sheets of 
metal, and afterward formed and tempered. Hook 
temples of steel are turned from wire upon a lathe. Bridges 
are usually made of oval or half-oval wire, and are simply 
pressed to the desired shape by a forming machine. 

Of the Different Patterns of Spectacles. — In the com- 
mon and strongest form of spectacle, the edge of the glass 



2 2 SPECTACLES AND EYEGLASSES. 

is beveled, so as to enter a groove in the wire which sur- 
rounds it. In a second form, in which the edge of the glass 
is grooved for the reception of a fine, round wire, the object 
sought, of rendering the rim of the spectacles less conspicu- 
ous, is generally defeated by the fact that the glass must be 
made thicker than it otherwise need be, in order to give 
room for the groove on its edge. In concave glasses this 
is not the case, since the edge of the glass is here the thickest 

Fig. 6. 




part, and such glasses may sometimes be mounted in 
this way with advantage. In a third form, called "frame- 
less" spectacles (Fig. 6), the wire encircling the glass is 
dispensed with altogether, small holes being drilled through 
the glass near its edge for the accommodation of screws 
which fasten the bridge and temples in place. The advan- 
tages of this form are its beauty and inconspicuousness. 
It should never be prescribed for children, as it is quite 



GENERAL CONSIDERATIONS. 



2 3 



liable to break at the point where the glass has been drilled. 
The edges of these glasses should not be polished, but 
should be given a dull finish, otherwise they reflect the light 
disagreeably. 

Sides, or temples, have been variously constructed. 
Those having sliding and turn-pin joints are examples of 
antiquated forms. Those now used are the "hook," or 
"riding-bow,'' and the plain, "straight" temple. The 
former are to be preferred in all cases where the glasses 
are to be worn constantly or nearly so, and the latter for 

Fig. 7. 




X," " K," " Curl," and " Saddle " Bridges. 



those who wear glasses for near work only, and require to 
remove them frequently from the eyes. Hook temples are 
made in three lengths, designated as short, medium, and 
long. These are sufficient for all cases. 

Securing a proper fit in the bridge, upon which so much 
of the comfort and efficiency of spectacles depends, was a 
difficult matter until the ingenuity of Dr. Charles Hermon 
Thomas, of this city, suggested what is known as the 
"saddle bridge," which solved the problem. (See Fig. 7.) 
This bridge may be varied to suit every possible case, and 



24 SPECTACLES AND EYEGLASSES, 

is always to be preferred. The "K" bridge., formed of 
wires in the shape of the letter K, is allowable in some 
cases. The nearly similar "X" bridge allows the glasses 
to teeter, or see-saw across the nose, with the motions of 
the head. It is. however, the best form of bridge for re- 
versible glasses; that is. glasses for persons having sight 
in one eye only, who may have their distant glass set in 
one side of a frame and their near glass in the other. By 
using this bridge and "straight temples, or hook temples 
without a shoulder at the hinge, the spectacles mav be 
turned over so as to bring either lens before the wearer's 
seeing eye. The old-fashioned bridge., called the "'curl, " is 
unobjectionable for cases in which the bridge of the nose is 
prominent,, or for the spectacles of old people., who like to 
slip their glasses down toward the end of the nose. Xone 
of the forms mentioned, however,, has any advantage over 
the saddle bridge for any case. A small piece of cork is 
sometimes attached to the under side of the bridge where it 
comes in contact with the skin. It is unnecessary if the 
frames fit the face of the wearer properly. If it be desirable 
to remove all pressure from the bridge of the nose and trans- 
fer it to the sides, it is best done by soldering a pair of guards, 
similar to those used on eyeglasses, to the spectacle bridge. 
The earliest spectacles appear to have had round eyes. 
Various other shapes are occasionally seen, as octagon, 
oblong, etc. The oval has about displaced these anti- 
quated forms, and is made in sizes known to the trade in 
America as follows : 

TABLE II— SIZES OE EYES. 

Xo. oo. T^f by i J in. or 41 by 32 mm. large coq. size. 

" o. i^f by iyf in. or 39 by 3c mm. coq. 

" t. 1^- by i\ in. or 37 by 28 mm. standard large eye. 

" 2. i^f by |4 in. or 36 by 25 mm. standard E. G. size. 

" 3. if by 1^ in. or 35 by 26 mm. standard interchange. 

" 4. i|4 by fl in. or 34 by 25 mm. standard small eye. 

" 5. 1^ bv || in. or 32 by 23 mm. children's size. 



GENEEAL CONSIDERATIONS. 
Fig. 8. 



2 5 









Ovals Showing the Actual Size of Eyes According to Table II. 

3 



26 



SPECTACLES AND EYEGLASSES. 



AYhere glasses are used for near work only, the eyes are 
sometimes made of semi-oval shape, allowing the line of 
sight to pass over their upper, straight edge when the 
wearer views a distant object. These are known as " half." 
"pulpit," or "clerical" eyes, and are very convenient, 
especially to public speakers, as their name implies. They 
do not seem to me as well known or as generally used as 
they should be. 

Bifocal Glasses. — When glasses of different focusing 
power are required for distant and near vision, the trouble 



Fig. q. 






<0 



<o 



incident to frequent changing is obviated by '•bifocal" 
glasses. That is, the lower part of the spectacle eye, which 
is used for near work, is made to differ in focusing pow^r 
from the upper part, which is used for distant vision. Such 
bifocal glasses are also called Franklin glasses, from the 
philosopher who. as we have seen, invented them. 

The object sought may be attained in various ways. In 
the earlv Franklin glasses each eye contained two half-oval 
pieces, with their straight edges in apposition (A, Fig. 9). 
This has been improved upon by making the line of junc- 



GENERAL CONSIDERATIONS. 27 

tion a curved one, giving somewhat greater latitude of 
distant vision and rendering the glass more secure in its 
frame. The most modern and successful form of bifocal 
glasses is figured at D, Fig. 9. They are called " ce- 
mented " bifocals. To the back or front surface of the 
distance glass is cemented, by means of Canada balsam, a 
small lens whose strength, added to that of the distance 
glass, equals the glass required for near work. The upper 
edge of the supplemental lens should be ground as thin as 
possible in order to render it inconspicuous. A special 
grade of these lenses is made by a patented process by which 
the supplemental lens is ground very thin by fastening it to 
a block of glass instead of one of iron, and grinding the two 
pieces of glass away together. These spectacles are strong, 
light and handsome, and may readily be made in the 
frameless form like those represented in Fig. 6. For 
cylindrical lenses this arrangement is, moreover, cheaper 
than the others, since only the distance glass need have the 
cylinder ground upon it, the supplemental segment being a 
simple sphere. The changes in the correction for near 
which are likely to be needed from time to time are readily 
and cheaply effected in this form of bifocal glass. 

The shape and size of the supplemental segment may be 
varied to suit all exigencies of use or taste. Fig. 10 illus- 
trates some of the more common forms, of which B and C 
are the most useful. 

In still another form of bifocal glass the small supple- 
mental lens figured at D is countersunk, that is to say, is 
cemented into a corresponding concavity ground in the dis- 
tance glass. Or the distance glass may. be composed of 
two full-sized plano-convex lenses with their plane surfaces 
in apposition, each of these surfaces being ground out at iis 
lower part, so as to house the small supplemental lens 
between them. These two forms admit of a reduction of 
weight and the abolition of chromatic aberration in the 



28 



SPECTACLES AND EYEGLASSES. 



heavy glasses required in aphakia. To accomplish the lat- 
ter purpose the distance lens is made of crown glass and 
the supplemental lens of flint glass. In this form they are 
called achromatic bifocals. Their disadvantage lies in the 
expense of their manufacture. 

Fused bifocals are a variant of the countersunk supple- 
mental lens. In their manufacture a small lens of flint 
glass is let into a large lens of crown glass by counter- 
sinking, as described above. Instead of cementing the sup- 
plemental lens in position, however, the lenses are heated 

Fig. io. 





to the point of fusion of the glass, when its two portions 
unite. The surfaces of the glass are then reground. One 
surface of the small supplemental lens is exposed to the 
grinding and is reduced to the same curvature as the cor- 
responding surface of the main lens. The necessary differ- 
ence in the refraction of the upper and lower portions is 
dependent on the difference in index of refraction of the 
crowm glass of which the main lens is composed and the 
flint glass of the supplemental lens. 

Another improved form of bifocal glass is made in 
one piece by a special method of grinding which pro- 
duces two different spherical surfaces on one side of the 



GENERAL CONSIDERATIONS. 



2 9 



lens. This is an old idea, and the old form of the glass is 
shown at C, Fig. 9. This old glass is very faulty from a 
prismatic effect inherent in the method of manufacture, and 
never came into general use. The modern glass is made 
strongly concavo-convex, the posterior surface being — 6. D. 
The lower portion of the surface, however, is ground on a 
separate tool to some less degree of concavity. The poste- 
rior surface becomes, therefore, bifocal. The anterior sur- 
face is given any required convexity. For instance, if 
the posterior surface is ground — 6. D., with its lower por- 
tion — 4. D., and the anterior surface is made + 7. D., the 
result will be a bifocal glass whose upper portion will be 

Fig. 11. 




+ 1. D. and the lower portion + 3. D. Where a cylindrical 
element is required in this glass the anterior surface is 
given the toric form. The fused bifocal and this ground 
bifocal resemble each other quite closely in appearance. 
They contain no cement and the line between the two por- 
tions is quite inconspicuous. In point of efficiency and 
cost there is not much choice between them. In point of 
comfort and use to the wearer they have not any very great 
superiority over the cemented form costing approximately 
half as much and in which the near segments may be 
changed at slight expense. 

All bifocals have the inconvenience that in walking, the 
floor just in front of the patient's feet is not seen clearly 
because viewed through the near glass. Spectacles which 



3o 



SPECTACLES AND EYEGLASSES. 



revolve on the long axis of the " eye, " bringing the distance 
glass to the lower portion of the frame, have been contrived 
to overcome this difficulty, but they are cumbersome and, 
moreover, it requires more effort to effect the revolution of 
the glass than it does to bend the neck sufficiently to bring 
the upper segment into the line of vision when the ordinary 
-bifocals are worn. Some persons declare that they cannot 
^become accustomed to bifocals however well adjusted. 
Parallel, horizontal lines, as those of a staircase, are particu- 
4arly confusing, it being possible to see each line doubled if 
the junction of the two segments of the glass is placed just 

Fig. 12. 



opposite the pupil. Such persons may prefer having an 
"extra front" (Fig. n): that is, a second pair of spectacles, 
whose temples are replaced by short hooks, by means of 
which they are hung in front of the frame already upon the 
face. This is a rather clumsy device; less so, however, 
when the eyes of the extra front are made half oval instead 
of oval. 

Eyeglasses. — The increased popularity of eyeglasses 
has stimulated invention and they now present a greater 
variety of forms than do spectacles. Every part is sub- 
ject to such change and adjustment that it is quite possible 
to fit them to cases which were out of the question with the 
old forms. 



GENERAL CONSIDERATIONS. 

Fig. 13. 



3 1 




j ■■i 



Fig. 14. 




Forms of Rigid Fkamk or " Bar Spring " Eyeglasses. 



32 SPECTACLES AND EYEGLASSES. 

An eyeglass is held in place by the pressure of a spring, 
or springs, upon the sides of the nose. There are four 
methods of placing this spring and hence four radically 
different forms of eyeglasses. First, we have the old and 
familiar form of arched spring (Fig. 12) which allows 
the frame to "open" while the lenses remain in their orig- 
inal plane. Second, the spring may be placed in a plane 
approximately at right angles to that of the lenses and the 
frame opens by the outer ends of the lenses moving forward. 
(Fig. 13.) Third, the lenses are joined by two bars 
sliding over each other in connection with a spiral spring. 
The frame opens when the two lenses are drawn apart 
along the line of their long axes. (Fig. 14.) Fourth, 
the lenses are joined by a rigid bridge, like a spectacle, 
while the nose-pieces are directly connected with spiral 

Fig. ik. 




springs which press them to the sides of the nose. To 
open the frame special levers for the thumb and finger 
are provided by means of which the nose-pieces are pressed 
apart. (Fig. 15.) In another variety these levers are 
operated by pressing forward the outer ends of. the lenses. 
The first of these forms still holds its place as the most 
generally useful. It is simple and inconspicuous and the 
spring acts directly. For the majority of cases it is to be 
preferred. Its spring may be made light or heavy and may 
be offset from the plane of the lenses to accommodate over- 



GENERAL CONSIDERATIONS. 33 

hanging brows. The second form offers no marked su- 
periority over the first, from which it differs only in the 
direction of the action of the spring. This may make its 
hold more certain in a few cases, and occasionally persons 
who have tried both will prefer this form. The third and 
fourth forms have the same object in view, namely, to 
hold the lenses in position with the certainty of a spectacle 
frame, allowing none of that displacement of the axis of a 
cylinder, or base of a prism which is possible when they are 
joined by a yielding spring. The form designated in this 
description as "third," and which is usually called the 
"bar-spring" eyeglass and of which there are several 
variations (Fig. 14) has not met with much favor. They 
are heavy and cumbersome looking and there is apt to be 
lost motion between their sliding bars, which allows the 
displacement they are intended to prevent. The last of 
our four forms has more merit. Here, the junction be- 
tween the lenses is really rigid, and no alteration of their 
relation to each other can take place. It is possible, how- 
ever, for one lens to be displaced upward and the other 
downward, or for one to stand forward and the other back- 
ward. This frame contains two springs, instead of one as 
in the other forms, and weakening of one spring may cause 
one of these displacements. Moreover, the small spiral 
springs are not very durable. In spite of its limitations, 
however, this frame has distinct value in some cases, es- 
pecially where the lenses are heavy. 

Nose-pieces are furnished in many forms. The use of 
the old form (Fig. 2) which lies in the same plane as the 
lenses, is now quite limited. The "offset guard," which 
bears upon the nose posterior to the plane of the lenses is 
much more generally useful. The latter should certainly 
be preferred for any case in which the glasses are to be worn 
for more than a few minutes at a time. There are dozens 
of varieties of the offset guard, many of which exist only 



34 



SPECTACLES AND EYEGLASSES. 



for trade purposes and are advertised much beyond any 
peculiar merit which they possess. Usually, the fewer 
and simpler the parts of any implement the better. The 
best guard is one stamped from a single piece of metal. 
Rivets will frequently loosen or fall out, or the metal may 
split to a rivet hole. As for pivots in nose-pieces, they are 
a delusion. What we should seek is not a self-adjusting 
eyeglass, but one capable of wide adjustment, and which 
will keep the shape which we give it. To this end the 
metal should be tough and pliable, though possessed 
of a certain amount of rigidity. The bearing surfaces of 




Various Patterns of the Offset Guard. 

A." For shallow bridge, prominent eyes, flat forehead. 
B." For shallow bridge, prominent eyes and forehead. 
C." The guard used for the average case. 
D." Deep-set eyes, prominent nose and forehead. 
E." Same as "C," but for lowering glasses (for reading). 
F." Same as "B," but for lowering glasses (for reading). 
G." Same as"C," but somewhat smaller and neater, although having 
less bearing surface. 



nose-pieces may be covered with cork, shell or celluloid. 
They soon become greasy and slippery, and beside being 
thick and clumsy, are not durable. If one tries to mold 
such nose-pieces to a different shape, the cork or shell 
frequently breaks, or the rivets pull out. The later pat- 
terns are stamped from a single sheet of metal, without 
any covering for the bearing surfaces, which are corrugated, 
fenestrated, or divided into two or more portions to give 



GENERAL CONSIDERATIONS. 35 

a more clinging hold upon the skin. In adjusting these 
nose-pieces to the patient's face they may be shaped with 
the pliers with much greater freedom than can the other 
forms. 

'The "arm " or "foot " of the nose-piece is, in most forms, 
made in several lengths and shapes, for use on variously 
proportioned faces (Fig. 16). Much of the adjusting of 
eyeglasses is done by bending and twisting this arm. 

Studs of eyeglasses are made in about six different 
lengths, from i mm. to 6 mm. By means of these the 
intercentral distance of the lenses may be varied. In each 
of these lengths an offset stud is made, which may be 
used to place the lenses farther forward. Moreover, there 
are angular forms useful in tilting the lenses. 

Spectacles for Cosmetic Effect. — Something may legiti- 
mately be done, at times, in the way of improving the ap- 
pearance of a patient by the application of glasses. The 
blind whose eyes are not only sightless, but unsightly, very 
commonly hide them behind colored glasses. Neatly fit- 
ting spectacles with large eyes of ground glass render the 
appearance of such persons less lugubrious. When one 
eye is useless for vision, and at the same time small, and 
the orbit undeveloped, a gratifying improvement in the ap- 
pearance of the patient may be attained by placing before 
the shrunken eye a convex glass of sufficient strength to 
magnify it to the size of its fellow. The condition known 
as epicanthus can generally be removed by wearing eye- 
glasses whose nose-pieces draw just enough on the inner 
canthi to smooth out the offending fold of skin. As the 
subjects of epicanthus are generally flat-nosed, it may be 
necessary to furnish the eyeglasses with a pair of hook 
temples to keep them in place. Since operations for this 
disfigurement are so unsatisfactory, such an appliance is 
probably the best treatment we can advise in case the 
trouble is not outgrown. 



II. THE PRINCIPLES OF SPECTACLE 
FITTING. 

We have now to consider the essential principles of plac- 
ing glasses before the eyes. The usefulness of spectacles 
depends almost as much upon the fidelity with which these 
principles are carried out as it does upon a careful correc- 
tion of the errors of refraction. 

Centering and Decentering. — By the visual axis, or, in 
English, the line of sight, is meant a line from the yellow 
spot of the retina through the nodal point of the eye to the 
object sighted. 

By the principal axis of a lens we mean a line passing 
through the optical center of the lens (the thickest part, if 
the lens is convex; the thinnest if concave) at right angles 
to its surfaces. 

The geometrical center of a spectacle glass may be 
shortly said to be that point on its surface which is equally 
distant from the extremities of the figure to which it is cut. 
The principal axis of the lens may or may not pass through 
this latter center. 

We habitually regard as the normal position for glasses 
one in which, when the eyes are looking at a distant ob- 
ject, the visual axes correspond exactly in position with 
the principal axes of the lenses, and together they pass 
through the geometrical centers of the spectacles. In 
other words, the geometrical center of the spectacle eye 
and the optical center of the spectacle lens coincide, and 
the center of the pupil for each eye lies directly behind 
them. Regarding decentering, some confusion is apt to 
arise because the word is used in two different connections. 

36 



THE PRINCIPLES OF SPECTACLE FITTING. 37 

If the visual axis pass to the temporal side of the optical 
center of a glass held before an eye, then, with respect to 
that eye, the glass is said to be "decentered in." If the 
visual axis pass to the nasal side of the optical center of the 
glass, the latter is " decentered out." Similarly a glass may 
be decentered in any other direction. When speaking of 
spectacles, however, without reference to the eyes of the 
wearer, they are said to be "decentered in" when their 
optical centers lie to the inner side of their geometrical 
centers; "decentered out" when the optical centers are 
to the external side of the geometrical centers, etc. A 

Fig. 17. 




Spectacles with Lens Decentered In. 
G G show the position of the geometrical centers; O O, that of the optical centers. 

glance at Fig. 17, which represents a pair of spectacles 
decentered in, will make clear what is meant. 

From the above it will readily be seen that when it is 
desired that a patient wear decentered lenses, the effect 
may be obtained in either of the two ways ; first, by decen- 
tering the lenses in their frame; second, by displacing 
them, together with their frames, from what I have de- 
scribed as the normal position. The first method has 
the disadvantage of increasing the weight of the glass, 
while the second limits the field of binocular vision. In 
practice, the second method should be employed to the 
greatest extent possible without unduly interfering with 
binocular vision for the distance at which the spectacles 
will be used, and, should still farther decentering be re- 
quired, the method first mentioned should be brought 



38 



SPECTACLES AND EYEGLASSES. 



into service. For instance, suppose we wish to order 
glasses with each lens decentered in 8 mm. This would 
mean that the optical centers are to be 16 mm. nearer 
together than the patient's pupils. Let us suppose that by 
a careful consideration of the distance for which the glasses 
are prescribed, of the distance at which they must be 
placed in front of the eyes, and of the size of the spectacle 
eye used, we find that the frame can only be made 10 mm. 
narrower than normal without the outer rims of the "eyes" 
becoming annoying. This leaves 6 mm. to be obtained by 
decentering the glasses in their eye wires. If the distance 
between the patient's pupils were 6o mm., we would order 
the distance between the geometrical centers of the spec- 
tacle eyes to be 50 mm., and each eye to be decentered in 
3 mm. 

Fig. 18. Fig. 19. 





Showing the Prismatic Effect of Decexterixg. 

The optical center, O, in Fig. 18 coincides with the geometrical center, G. In Fig. 19, 
which represents a decentered lens of the same spherical curvature, O has been 
removed toward the base of the virtual prism b a c. (After Maddox.) 

Prismatic Effect of Decentering. — It is to obtain a pris- 
matic effect from spherical lenses that decentering is gener- 
ally ordered, since a decentered lens is identical with a 
lens of the same strength combined with a prism. This 
is graphically shown by Figs. 18 and 19, the latter of which 
represents a section of a decentered lens, which will readily 
be seen to be precisely the same as the result would be 
if the normally centered lens shown in Fig. 18 were split 
into halves and the prism b a c introduced between them. 

The size of the glass disk from which spectacle lenses are 



THE PRINCIPLES OF SPECTACLE FITTING. 39 

ground Will not allow of more than about 2 mm. of lateral 
deeentering for a No. 1 eye; 3 mm. for Nos. 2 and 3; and 
4 mm. for No. 4. Vertically, they may be decentered 
much more. When ordered to decenter laterally more 
than this, or to furnish a prismatic effect greater than 
can be obtained by this much deeentering, the optician 
first manufactures a prism of the requisite strength, and 
then grinds spherical surfaces upon its two faces. It is, 
therefore, of not much importance whether, in ordering a 
sphero-prismatic combination, we express the prismatic 
element in degrees of the refracting angle, or in millimeters 
of decentration of the lens: the optician produces the glass 
by whichever method is the more convenient. 

The stronger the lens, the less deeentering it requires to 
produce a given prismatic effect, and where the combina- 
tion desired is that of a strong lens with a weak prism, the 
more accurate practice probably is to order the lens decen- 
tered the requisite number of millimeters. For this pur- 
pose a table of equivalents, such as is given below, is neces- 
sary. To use it we find in the first column the strength 
of the lens used, and on a level with this, in the column 
at whose head stands the strength of the prism required, 
is given in millimeters the amount of decentration neces- 
sary. 

It is one of the beauties of the reformed numbering of 
prisms (see page 16), that by a simple calculation one can 
tell in a moment the amount of decentration required to 
produce any required number of centrads, by means of any 
given lens. 

Divide the number of centrads required by the strength 
of the lens, in diopters. The quotient is the necessary de- 
centration, in centimeters. For example: to produce a 
prismatic effect of 3. Cr. by means of a lens of 5. D., it is 
necessary to decenter as many centimeters as 5 is contained 
times in 3, which is .6 centimeters. 



40 



SPECTACLES AND EYEGLASSES. 



TABLE III.*— DECENTERING EQUIVALENT TO A GIVEN RE- 
FRACTING ANGLE (INDEX OF REFRACTION, 1.54). 



Lens 


i° 


2° 


3° 


4° 


5° 


6° 


8° 


IO° 


1 D, 


9.4 


l8.8 


28.3 


37-7 


47.2 


56.5 


75-8 


95-2 


2 


4-7 


9.4 


14.1 


18.8 


23.6 


28.2 


37-9 


47.6 


3 


3-i 


6-3 


9.4 


12.6 


i5-7 


18.8 


25-3 


3i-7 


4 


2 -3 


4-7 


7-i 


9.4 


11. 8 


14. 1 


18.9 


23.8 


5 


1.9 


3-8 


5-7 


7-5 


9.4 


n-3 


15.2 


T 9 


6 


1.6 


3-i 


4-7 


6-3 


7-9 


9.4 


12.6 


15-9 


7 


i-3 


2.7 


4 


5-4 


6.7 


8.1 


10.8 


13-5 


8 


1.2 


2 -3 


3-5 


4-7 


5-9 


7-i 


9-5 


11. 9 


9 


1 


2.1 


3- 1 


4.2 


5-2 


6-3 


8-4 


10.5 


to 


•9 


1.9 


2.8 


3-8 


4-7 


5-6 


7.6 


9-5 


11 


•9 


i-7 


2.6 


3-5 


4-3 


5-i 


6.9 


8.7 


12 


.8 


1.6 


2.4 


3-i 


3-9 


4-7 


6-3 


7-9 


13 


•7 


1.4 


2.2 


2.9 


3-6 


4-3 


5-8 


7-3 


14 


•7 


i-3 


2 


2.7 


3-4 


4 


5-4 


6.8 


15 


.6 


i-3 


1.9 


2-5 


3- 1 


3-8 


5- 1 


6-3 


16 


.6 


1.2 


1.8 


2.4 


3 


3-5 


4-7 


6 


17 


.6 


1.1 


i-7 


2.2 


2.8 


3-4 


4-5 


5-6 


18 


•5 


1 


1.6 


2.1 


2.6 


3- 1 


4.2 


5-3 


19 


•5 


1 


i-5 


2 


2.5 


3 


4 


5 


20 


•5 


•9 


1.4 


1.9 


2.4 


2.8 


3-8 


4.8 



Table IV is constructed by applying this rule. In it, 
however, the distances which the lenses must be decentered 
have been reduced to millimeters by moving the decimal 
point one place to the right, in order to make it practically 
more convenient, and render it homologous to Table III, 
like which it is used. 

A cylindrical lens, or the cylindrical element of a sphero- 
cylindrical lens, when decentered in a direction vertical to 
its axis, acts as a spherical lens of the same strength. 
Thus, a + 2. Sph. O +1. Cyl. axis vertical, decentered 
horizontally, would have the same prismatic effect as a +3. 
Sph. treated in the same way. As the axis is inclined 
toward the direction of decentration, the prismatic effect of 



* Jackson: 
1889. 



: Transactions of the American Ophthalmological Society. 



THE PRINCIPLES OF SPECTACLE FITTING. 



41 



the cylinder diminishes, and disappears when they coin- 
cide. Thus, a + 2. Sph. O + 1. Cyl. axis horizontal, de- 
centered horizontally, would have merely the prismatic 
effect of a + 2. Sph. so treated. 



TABLE IV.— DECENTERING EQUIVALENT TO A GIVEN NUM- 









BER OF CENTRADS. 








Lens 


1 Cr. 


2 Cr. 


3 Cr. 


4 Cr. 


5 Cr. 


6 Cr. 


8 Cr. 


10 Cr 


1 D, 


10 


20 


30 


40 


5° 


60 


80 


100 


2 


5 


10 


^5 


20 


25 


3° 


40 


5° 


3 


3-3 


6.6 


10 


13-3 


16.6 


20 


26.6 


33-3 


4 


2-5 


5 


7-5 


10 


12.2 


15 


20 


25 


5 


2 


4 


6 


8 


10 


12 


16 


20 


6 


1.6 


3-3 


5 


6.6 


8-3 


10 


13-3 


16.6 


7 


1.4 


2.8 


4.2 


5-7 


7-i 


8.2 


11.4 


14.2 


8 


1.2 


2-5 


3-7 


5 


6.2 


7-5 


10 


I2 -5 


9 


1.1 


2.2 


3-3 


4.4 


5-5 


6.6 


8.8 


11. 1 


10 


1 


2 


3 


4 


5 


6 


8 


10 


11 


•9 


1.9 


2.8 


3-7 


4.6 


5-5 


7-3 


9 


12 


.8 


1.8 


2-5 


3-3 


4.1 


5- 


6.6 


8-3 


13 


,*7 


i-5 


2-3 


3 


3-8 


4.6 


6.1 


7.6 


14 


•7 


[.4 


2.1 


2.8 


3-5 


4.2 


5-7 


7-i 


15 


.6 


t-3 


2 


2.6 


3-3 


4 


5-3 


6.6 


16 


.6 


1.2 


1.8 


2-3 


3-i 


3-7 


5 


6.2 


17 


•5 


1.1 


i-7 


2-3 


2.9 


3-5 


4-7 


5-8 


iS 


•5 


1.1 


1.6 


2.2 


2.7 


3-3 


4.4 


5-5 


IQ 


•5 


1 


i-5 


2.1 


2.6 


3-i 


4.2 


52 


20 


•5 


1 


i-5 


2 


2 -5 


3 


4 


5 



Normal Lateral Centering. — In proportion as the pris- 
matic effect of decentered lenses is a valuable property 
where this effect is desired, it has to be guarded against in 
those cases which do not require it, to which number 
belong, of course, the great majority of the cases we are 
called upon to treat. If the objects looked at through 
spectacles were always situated in the same direction and 
at the same distance, fixing the position proper for the 
centers would be a simple matter; but, in the movements 
4 



42 SPECTACLES AND EYEGLASSES. 

of the eyes, each pupil roves over a territory some 18 mm. 
(J in.) long by 15 mm. broad. When the eyes are directed 
toward a distant object the centers of the pupils are about 
60 mm. apart, and on convergence only 56 mm., so that 
the proper adjustment of spectacles is a series of compro- 
mises between that proper for the position of the eyes in 
which the glasses will be most used and other positions in 
which they will be less used. Of course, the position in 
which they will be most used must receive the greatest 
consideration. 

The proper position for the centers of " distance' ' glasses 
has already been stated. When glasses are to be used for 
near work only, they should be decentered "in" two or 
three millimeters on each side from this " normal '" position, 
as such glasses, being never used in that position, but only 
when the visual axes are converged, would otherwise never 
be rightly centered. What amounts to the same thing, and 
is more often done, is to make the front of the near specta- 
cles four or six millimeters narrower than if they were 
intended for distant vision : four millimeters narrower for a 
working point of 15 inches; six millimeters narrower for 
one of 10 inches. Concerning the centering of glasses 
which are worn constantly, no rule for all cases can be laid 
down, since accurately centering for any one distance is 
decentering for every other. Fortunately, as a glance at 
Table III will show, it is only with lenses of high power 
that a considerable amount of prismatic effect is developed 
by slight decentering. Where such glasses must be worn 
constantly by a person who spends several hours daily at 
near work, they should certainly be slightly decentered 
inward. 

The distance between the geometrical centers is regulated 
by the size of the spectacle eyes and the width of the space 
between them occupied by the bridge. Where the inter- 
pupillary distance is short, as in children, opticians are apt 



THE PRINCIPLES OF SPECTACLE FITTING. 43 

to make the eyes of the spectacles so small as to interfere 
seriously with the field of vision through them. With the 
saddle-bridge there is no difficulty in diminishing the space 
between the spectacle eyes without interfering with the 
form of that part of the bridge which is applied to the 
nose, and the required adjustment should be made in this 
way, leaving the spectacle eyes of good size. 

Normal Vertical Centering. — The glasses require, fur- 
ther, to be so placed that the points where the wearer's 
visual axes penetrate them shall neither be above nor below 

<* Fig. 20. 



the centers. This adjustment is readily seen to depend 
upon the relative height of the bridge of the spectacles and 
the bridge of the nose at the point where the spectacles 
rest. The higher the spectacle bridge, the lower will the 
glasses stand upon the patient's face, and vice versa. 

On the bridge of nearly every nose there may be felt a 
point at which the narrow, upper portion of the nasal bones 
gives place rather suddenly to the broader lower portion. 
Just here, in what has been called the "natural" position 
(a, Fig. 20), the bridge of the spectacles tends to rest, and 
the attempt to make it remain at any other point will not 
be very successful. In distance spectacles, then, the bridge 



44 



SPECTACLES AND EYEGLASSES. 



should be made of such height that when resting at this 
natural position, the centers of the spectacle eyes are at the 
same height as the centers of the pupils when the patient 
looks straight forward. When the glasses are to be used 
for near work only, their bridge should be made about 2 
mm., or J inch, higher than otherwise, allowing the centers 
to drop that much lower, as the wearer's eyes will nearly 
always be directed to objects below their own level. 

Distance of the Glasses from the Eyes.— As a rule, the 
glasses should be placed just far enough from the eyes 
to escape the lashes in the act of winking. If the lashes 
touch the glass the latter quickly becomes soiled, and to the 

Fig. 22. 



Fig. 




( 



,..•••' 



spectacles is, moreover, attributed any falling out of the 
lashes which may occur. Some persons, however, with 
myopia of high degree, prefer the glasses to be placed as 
close to the eyes as possible, regardless of the lashes, be- 
cause of the larger clear images which they thus obtain. 
This adjustment of the glasses depends upon the relation of 
the top of the spectacle bridge to the plane of the glasses. 
Where the eyes are deep set, or the nose of the aquiline 
type, the top of the spectacle bridge must be in front of the 
plane of the glasses, or, as it is shortly called, "out" 
(Fig. 21). When the bridge of the nose is low and the 
eyes relatively prominent, as in the negro, Chinese, and 
children, the top of the bridge must be back of the plane 
of the glasses, or "in," as represented in Fig. 22. 



THE PRINCIPLES OF SPECTACLE FITTING. 



45 



Perpendicularity of the Plane of the Lenses to the Vis- 
ual Axis. — A very important requirement, and one not 
sufficiently regarded in the fitting of frames, is that the 
plane of the correcting lens when in use shall be as nearly 
as possible perpendicular to the visual axis. The stronger 
the lens the more important is this detail, whose warrant 
lies in the fact that the refractive value of a given lens 
placed obliquely to the visual axis is no longer that indi- 
cated by its number, but is that of some other, stronger 
lens. A cylindrical lens so placed acts simply as a stronger 
cylindrical lens; a spherical lens, however, as a stronger 
spherical lens combined with a cylindrical lens with its 
axis at right angles to that about which the lens is rotated. 

The results of the investigations of himself and others, of 
the effect of the obliquity of a lens to an incident pencil of 
rays, was summarized by Dr. Edward Jackson in a paper 
read before the American Medical Association in 1877, an d 
their practical application to this part of our subject pointed 
out. From that communication the following table is 
extracted. It gives in the first column the degrees of 
obliquity at intervals of 5 up to 45 . In the second col- 
umn is shown the refractive value of a 1. D. cylindrical, in 
the third that of a 1. D. spherical lens so inclined. 

TABLE V. 



Obliquity 


Refractive Power of a 


Sphero-Cylindrical Equivalent 


of 


1. D. Cylindrical 


of a 1. D. Spherical Lens 


the Lens. 


Lens so Placed. 


so Placed. 


o° 


1. D. cyl. 


1. D. spherical. 


5° 


t.OI f< 


1. 00 sph. 00.01 cyl. 


IO° 


1.04 " 


1. 01 sph. 00.03 cyl. 


iK° 


1. 10 " 


1.02 sph.O0.08 cyl. 


20° 


1. 17 " 


1.04 sph.O0.13 cyl. 


25° 


1.30 " 


1.06 sph.O0.24 cyl. 


30 


1.4.4 " 


1.09 sph.O0.36 cyl. 


35° 


1.69 " 


1. 12 sph.O0.56 cyl. 


40° 


2. 01 " 


1. 1 6 sph.Oo. 83 cyl. 


45° 


2.46 '' 


1.22 sph.O1.24 cyl. 



46 



SPECTACLES AND EYEGLASSES. 

Fig. 23. 




Fig. 24. 




THE PRINCIPLES OF SPECTACLE FITTING. 47 

To fulfil this requirement of perpendicularity to the 
visual axis, the lenses of spectacles used only for distance 
should lie in a vertical plane; that is, they should face 
directly forward, as shown in Fig. 23. Since the visual 
axes are directed downward and forward when near work is 
done below the level of the eyes, glasses for near must 
face downward and forward, as in Fig. 24, in order that 
the plane in which they lie shall be perpendicular to those 
axes. Furthermore, in viewing near objects the visual axes 
are directed inward and toward each other. This will re- 
quire the glasses to face inward also, as represented in Fig. 
25, so that they come to lie in different planes, instead of in 
the same plane, as formerly. 

Fig. 25. 




When "constant" glasses are prescribed, the lenses 
should be placed midway between the proper facing for 
near and that for distance glasses. Then, though the lens 
is not exactly properly inclined either for distant vision or 
near work, the result of such slight obliquity to the visual 
axis is unimportant, since, as a reference to Table V will 
show, it is only in the higher degrees of obliquity that the 
increase in power, and especially the development of cylin- 
drical effect from spherical lenses, is rapid. Moreover, by 
slightly bending the neck a moderate degree of obliquity 
of the glasses to the visual axis may be removed without 
discomfort to the wearer. 

The position of bifocal glasses should also be between 
that proper for near and for distance glasses, but nearer 



48 SPECTACLES AND EYEGLASSES. 

that of the stronger glass. This will generally be the near 
glass, as convex bifocals are much more frequently pre- 
scribed than concaves, and such glasses should face only a 
little less downward than glasses intended entirely for near 
work. When concave bifocals are worn, however, they 
should face more forward and much less downward. 

The angle which the plane of the glasses makes with the 
plane of the wearer's face depends entirely upon the angle 
formed by the plane of the glasses and the temples of their 
containing frames. Thus, when the temples are perpen- 
dicular to the plane of the glasses, as in Fig. 23, the latter 
will face forward and not at all downward. They may be 
made to face downward to any required degree by simply 
turning down the temples at the points where they are 
hinged to the end pieces. These must be equally turned 
down, however, as where only one is turned down, or one 
more so than its fellow, the result is not to make the glasses 
face downward, but to make the glass on the side of the 
lower temple ride higher on the face than its fellow. 

Periscopic Glasses. — In the effort to further apply the 
law requiring that the plane of the lenses shall be perpen- 
dicular to the visual axes, we are met with the fact that 
with biconvex and biconcave lenses this relation is only 
strictly possible within a comparatively limited area sur- 
rounding the optical center of the lens. When the wearer 
looks through the periphery of his glasses the visual axes 
will pierce the lenses obliquely, and the refractive value of 
the latter will, of course, be governed by all the laws of 
tilted lenses. For instance, when the wearer of an ordinary 
convex lens looks through it near the edge, the optical 
effect of the glass before his eye is that of a stronger con- 
vex lens combined with a cylindrical lens; the axis of the 
latter depending on the part of the periphery pierced by 
the line of sight. In weak lenses, the slight inaccuracy 
of vision produced in this way is of small moment, but 



THE PKINCTPLES OF SPECTACLE FITTING. 49 

where the strength of the lens used is greater than about 
2. D. the patient's field of accurate vision is greatly reduced 
in size, and in viewing objects not directly in front of him 
he is obliged to perform wide motions of the head in order 
to be able to see them through the central portion of his 
glasses. This is especially true of cases of aphakia, where, 
of course, very strong lenses are generally necessary. To 
escape or lessen these disadvantages, strong spherical 
lenses should be, and generally are, made in the form of a 
meniscus, which when placed with its convex surface from 
the eye constitutes a periscopic glass. The ideal of this 
form of lens may be defined as a glass in which the center 
of curvature of one surface coincides with the center of 
rotation of the eye, and that of the other surface approaches 
it as closely as the required strength of the glass will per- 
mit. In such a glass the visual axis will always be perpen- 
dicular to the first surface, and nearly so to the second, at 
whatever point it pierces the glass, and in whatever direc- 
tion the eye may be turned. 

When a cylindrical or sphero-cylindrical lens is required, 
the best form of glass is the toric lens described on page 
20. These lenses have, however, never been manufactured 
extensively, and the process of their manufacture, as well 
as the lens itself, being patented in this country, their cost 
is considerable. By transposing the usual formula, how- 
ever, there may be obtained from any optician a sphero- 
cylindrical lens which approaches the periscopic form, and 
is certainly superior to one ground after the usual method. 
For illustration, if one desires to order + 2. D. Sph. O + 
.75 D. Cyl. Ax. 90 , the formula may be transposed and 
the order written for -f 2.75 D. Sph. O — -.75 D. Cyl. Ax. 
180 . This glass, though optically of the same strength as 
the first, would have an approach to the periscopic form 
if placed with the cylindrical surface next the eye. The 
field of accurate vision would gain in all directions, espe- 
5 



50 SPECTACLES AND EYEGLASSES. 

cially in the vertical one, in which diameter, however, its 
enlargement is not of so much consequence as it is later- 
ally. Aphakic eyes offer the best field of usefulness for 
this practice, as in them we have generally to deal with a 
high hyperopia, and often with hyperopic astigmatism re- 
quiring for its correction a convex cylinder with its axis 
horizontal. Let us suppose that after a cataract extraction 
we wished to order + 10. D. Sph. O + 6. D. Cyl. Ax. 180 . 
With this lens accurate vision would be limited to a vertical 
oval field situated directly in front of the patient, beyond 
the confines of which all objects would appear distorted by 
various cylindrical effects. We would, therefore, transpose 
the formula into + 16. D. Sph. O — 6. D. Cyl. Ax. 90 , 
and this glass will be likely to give the patient much more 
satisfaction than the other would have done, as with it he 
obtains a very good lateral field. 



III. PRESCRIPTION OF FRAMES. 

In order to prescribe the frames for a pair of spectacles, 
we must, after measuring the face or a frame which fits, re- 
cord the dimensions of the frame we desire to order. The 
essential measurements are the intercentral distance, or 
width of front, and the three dimensions of the bridge. 
This list may be extended to include the measurement of 
the angle formed by, the crest of the bridge and the plane 
of the lenses, that formed by the temples and the plane of 
the lenses, the distance between the temples an inch back 
of the glasses, and the distance from the hinge of the tem- 
ples to the top of the wearer's ear. All these details are, 
however, so ready of adjustment, and the trouble and un- 
certainty of their prescription are so great, that in my judg- 
ment they are better left until the frame is received from 
the maker and we are ready to adapt it to the patient's face. 
The distance between the centers of the spectacle eyes is 
best obtained by measuring upon the face the distance be- 
tween the centers of the pupils; the other dimensions of 
the frame, however, are more easily obtained by trying on 
a sample frame and taking the measurements from this, 
estimating any change which may be necessary. To do 
this requires a half dozen sample frames of different dimen- 



Size of eye. 


Between 
centers. 


Height of 
bridge. 


Top of bridge 
in or out. 


Width of 
base. 


Length of 
temple. 


No. oo 
o 

i 
I 

2 

3 


66 mm. 
64 
62 
60 

57 
55 


8 mm. 
6 

4 

2 

3 
1 


1 mm. out 

2 " 
2 

2 in 

1 




20 mm. 

24 
21 

IQ 
16 

15 


Ions: (6^ in.) 
medium (6 in.) 

short (5 ] in.") 



51 



52 



SPECTACLES AND EYEGLASSES. 



sions in their different parts. With such an assortment as 
is here given, which is a very good one, a part nearly or 
exactly of the required size can always be found in one or 







PRESCRIPTION OF FRAMES. 53 

the other of the frames. It may, of course, be necessary 
to get the height of bridge from one frame, its breadth 
from another, and the length of temple from still a third. 

A rule graduated in millimeters or sixteenths of an inch 
is also necessary. 

I have had made for this purpose a rule which I think 
facilitates the work. As represented in Fig. 26, it has 
upon one side three scales graduated in millimeters and 
conveniently placed for taking the different dimensions of 
the frame, while on the reverse side are several ovals show- 
ing the principal sizes of spectacle eyes. Some of the uses 
of these scales are shown in Figs. 27, 28, 33, and 34; to 
avoid confusion one scale only is drawn in each diagram. 



Name of Patient, .... 


Philadelphia,.... 






.190 


0. D 


O. 5 


Unless otherwise specified, 
saddle bridge ; No. 2 eyes. D 
sions given are in millimeters 

Frames of * , 

Inter pupillary Distance... 

(Height 

Bridge < . 

(Width of Base... 


: urnish the followir 
lmensions are giver 

Catalogue 


ig: Medium length temples; 
to middle of wires. Dimen- 

No • 

r . in 
Top 


r out 














...M. 


D. 



54 SPECTACLES AND EYEGLASSES. 

A prescription blank such as that here given indicates 
what measurements are required, and will be found useful 
in practice. ' The upper part is for the lenses, the lower 
part for the frames. 

To Obtain the Interpupillary Distance, with which 
the first dimension of the frame, the distance between the 
geometrical centers (A to B, Fig. 27) is generally identical, 
the physician seats himself facing the patient in a good 
light, the latter being directed to look straight before him 
at some distant object. The measuring rule is placed be- 
fore the patient's eyes, as close to them and as far from the 
physician's eyes as possible. The zero of the scale being 



Fig. 2' 




placed opposite the center of one pupil, the center of the 
other may be marked by the physician's thumb nail, as 
represented in Fig. 28, and the distance between them read 
off the scale. This distance seldom varies more than 5 mm. 
from 60 mm., or 2 J in. It will be observed that as the 
physician's eyes are less than the length of his arm away 
from the patient's face when this measurement is taken, 
in fact, about two feet away, the marks upon the rule, though 
apparently opposite the pupils, will in reality be a little 
within the centers; so that the distance obtained will be a 
little less than it should be. When the physician's eyes 
are two feet away from those of the patient, and the rule 
is one inch away from them, the error in measuring an 
interpupillary distance of 60 mm. by this method is almost 
exactly 2 mm. This amount should, therefore, be added 



PRESCRIPTION OF FRAMES. 



55 



to the apparent interpupillary distance to obtain the true 
one. 

The measurement obtained in this way is sufficiently 



Fig. 2 




accurate for most purposes, but if a greater degree of accu- 
racy be desired in any case it may be attained by means 



Fig. 30. 



Fig. 92. 




Dr. Maddox's Pupil Localize! 




The Pupil Localizer in Use. 



of the little device suggested by Dr. Maddox, which is 
represented in Fig. 29. This is to be placed before one of 



56 



SPECTACLES AND EYEGLASSES. 



the patient's eyes in an ordinary trial frame having a grad- 
uated bar for showing the distance of each geometrical 
center from the middle of the bridge. The gaze of the 
observed and that of the observing eye being directed to 
each other's pupils, the two sights of the implement are 
brought into line between them as shown in Fig. 30. The 

Fig 31. 




same procedure is then gone through with for the other 
eye, and the distance of the second pupil from the median 
line of the face, as registered by the trial frame is added to 
that of the first, to obtain the interpupillary distance. 
This r procedure is also of advantage in revealing and meas- 
uring any difference in the distance of the pupils from the 



PRESCRIPTION OF FRAMES. 57 

median line, due to asymmetry of the face. The use of a 
trial frame for making accurate measurements requires the 
bestowal of considerable attention to see that the support 
of the nose-piece is vertical, the joints close and tight, and 
the markings correct ; otherwise it may readily introduce 
the errors its use is intended to obviate. There are, in the 
shops, many special forms of the ' ' pupillometer ' ' constructed 
on the principle of a rule held before the eyes and a single 
sight for each pupil. Two of these are shown in Figs. 31 
and 32. The interpupillary distance as registered by them 
requires, of course, the same correction as does that ob- 
tained by the simple graduated rule. 

Height of the Bridge. — This is the distance of the top 
of the bridge above a line joining the centers of the lenses. 

Fig. 32. 




x m ,t • ' ' '' H" 



In Fig. 27, it is the distance from K to F, which is the 
height of K above a line joining a and b; not the height 
of K above a line joining c and d, which is sometimes 
erroneously supposed to represent the height of the bridge. 
If a rule be held horizontally before the patient's eyes, 
wdth the lower edge touching the nose at the natural posi- 
tion for the spectacle bridge, the height of this edge of the 
rule above the pupil on either side will show at a glance 
about how high the top of the future bridge must be. We 
may then select from our sample frames that one whose 
bridge corresponds most nearly with this supposed height, 
and being sure to place it in the natural position, we care- 
fully note whether the pupils are above or below the 
centers of the eyes of the frame. If they are below these 
centers, sufficient must be added to the height of the bridge 



58 SPECTACLES AND EYEGLASSES. 

now upon the face to allow them to coincide; if the pupils 
are above the centers, a corresponding subtraction from 
the height of the trial bridge must be made. Each sample 
frame may have its dimensions attached to it, or any frame 
may be used as a fitting frame and afterward measured. 
To measure the height of a bridge the glasses are laid 
upon a sheet of ruled paper, or other object offering a con- 
venient straight line, in such a way that the line passes 
through the geometrical centers of the eyes, or, what is the 
same thing, through the joints of the end pieces on each 
side (Fig. 27). The height to which the bridge projects 
above this line is then readily measured. It is seldom 

Fig. 33. 



Millimete r Scale. 




greater than 10 mm., and in rare cases may be a minus 
quantity, the top of the bridge being below the level of 
the centers of the lenses. 

Relation of the Top of the Bridge to the Plane of the 
Lenses. — The measurement required to express this 
relation is that from J to K in Figs. 33 and 34; not the 
distance of / in front of a line joining C and D, as might 
be supposed. This measurement is also shown at H I, 
Figs. 21 and 22; it is obtained by a procedure similar to 
that just described for obtaining the height of the bridge. 
The rule being placed across the nose at the natural point, 



PRESCRIPTION OF FRAMES. 59 

and the patient requested to wink, it may readily be seen 
whether the lashes touch the edge of the rule. If they do, 
the top of the bridge of the future spectacles must be back 
of the plane of the glasses, or "in." If they do not, we 
note how much nearer, if any, the edge of the rule might 
be brought without their touching, and so obtain a guide to 
the distance the top of the bridge sliould be in front of the 
plane of the lenses, or "out." The fitting frame which 
comes nearest to the requirements of the case in this partic- 
ular is then placed upon the face, when by viewing it from 
above or from the side it can quickly be seen just how 
much change, if any, is needed to place the glasses a little 
beyond the reach of the lashes. The method of measur- 
ing the distance of a bridge in or out is so plainly shown in 

Fig. 34. 




Figs. 33 and 34 that special explanation is unnecessary. 
They seldom measure more than 4 mm. out or 3 mm. in. 

Width of Base. — The measurement from C to D, Fig. 
33, is obtained, like the others, by measuring a bridge 
which fits, or estimating the change necessary in one which 
does not. This dimension is usually from 16 mm. to 20 
mm. 

Angle of the Crest of the Bridge. — It is not usually 
necessary to prescribe this angle, for the reason that the 
ine formed by the bones of the nose, as seen in the pro tile 



60 SPECTACLES AND EYEGLASSES. 

of the face, is nearly always vertical at its upper portion and 
its direction changes so as to approach the horizontal as the 
nasal bones expand and jut forward to form the bridge of 
the nose. The direction of some portion of this line will 
coincide with the flat under surface of the top of the spec- 
tacle bridge, and it is on that portion of the line that the 
spectacles will tend to rest. There are cases, however, of 
noses with a very straight and vertical outline, in which the 
flat wire forming the top of the spectacle bridge finds no 
suitable support and rests, more or less, on its posterior 
edge. In other cases, exaggerations of the aquiline type, 
the line of the crest of the nose turns sharply forward to 
a nearly horizontal direction and the wire of the bridge 
tends to rest on its anterior edge. In these cases it is well 
to prescribe the angle which the top of the bridge makes 
with the plane of the lenses. Mr. Merry, of Kansas City, 
has invented a little implement for quickly determining this 
angle. Its appearance and the manner of its use are so 
well shown in Figs. 35 and 36 as to require no description. 

This method of obtaining the dimensions of the bridge 
required may seem tedious and uncertain in the description ; 
in use it is not so, and after trial I think will be found 
preferable to any special device so far invented for record- 
ing the measurements. These, after shifting of screws and 
bending of wires, leave one to estimate what changes are 
required just as might have been done without their aid. 
Moreover, the heavy parts and lost space in joints of trial 
frames may readily conceal an error of 2 mm., or even 3 
mm. in some measurements; the large, round eyes with 
heavy rims will not go under the brows, so that the in-out 
measurement of the bridge must frequently be guessed at; 
and the relation of the upper part of the eye wires to the 
brows is not shown. In fact, they introduce, in my estima- 
tion, quite as many sources of error as they eliminate. 

Where the face is unsymmetrical no exact rules of pro- 



PRESCRIPTION OF FRAMES. 



6l 



cedure can be given, and considerable ingenuity may be 
required to fit a frame to such a face. If the nose is very 
peculiar, or one side of its bridge markedly steeper than 
the other, it may be of advantage to take an outline of the 
bridge at the natural position by bending a piece of lead 
wire to fit accurately and marking, the outline of this upon 



^ggn 




B 



the prescription blank, or sending the wire itself to the 
spectacle maker. Sometimes the brows are overhanging 
and the eyes deep set ; so that the glasses cannot be prop- 
erly centered before the eyes and placed close to them 
without the upper part of the rims burying themselves in 
the brows. In such cases the glasses should be decentered 
upward in their frames and the bridge made sufficiently 



62 SPECTACLES AND EYEGLASSES. 

high to bring the optical centers opposite the pupils. 
Though the patient will then look through the upper part 
of his glasses, his field of vision will not be any more limited 
than is already the case because of the overhanging brows. 
Prescription of Eyeglasses. — The dimensions which it 
is usual to furnish in prescribing eyeglass frames are the 
interpupillary distance, of course, with the distance between 
the two upper and the two lower ends of the nose-pieces 
when they are in place on the face (.4 to B, and C to D, 
Fig. 12). These measurements alone will not insure a good 
fit in the frames, since neither the contour of the sides of 
the nose to which the guards are applied, the vertical cen- 
tering of the lenses, nor the distance of the latter from the 
eyes are taken into account; but the same remark applies 
here as to the minor dimensions of spectacle frames, namely : 
that it is more simple, certain, and expeditious for the 
surgeon to make these adjustments in the frames themselves 
than to prescribe what the manufacturer shall do for him. 
A series of three or four frames with variations in the length 
and shape of the spring and in the pattern of the guards is 
sufficient for trying on. Fortunately, eyeglass frames ad- 
mit of great variation by bending their different parts, and 
being put together with screws, these parts are quickly inter- 
changeable. Almost the only thing about them which ad- 
mits of no adjustment is the length of the spring, and it is 
well for one who prescribes many eyeglass frames to have 
a series of such springs at hand from which to replace one 
which may be found too long or too short. 



IV. INSPECTION AND ADJUSTMENT OF 
SPECTACLES AND EYEGLASSES. 

Ordinary prudence demands that the prescriber of 
glasses make a careful examination of the manner in which 
his directions have been carried out, since neglect of this 
precaution may nullify the results of the most painstaking 
correction of the refraction. If the surgeon himself furnish 
the spectacles, it is doubly incumbent on him to make a 
thorough inspection of glass and frame, and to carefully 
adjust the latter so as to be entirely comfortable to the 
wearer. Then, too, it is not enough that the frames cor- 
rectly perform their function at first; they must continue 
to do so. Should there be no optician in his neighbor- 
hood, the surgeon will be called upon to bring to a proper 
shape frames which have passed through all sorts of acci- 
dents, and it is better that he should do this work than 
entrust it to less competent hands. 

Proving the Strength of Lenses. — The focal length of 
a convex lens may be directly measured by finding the 
distance at which it brings the sun's rays to a focus. To 
do this, the rays which have passed through the lens are 
simply caught upon a piece of paper or other screen, the 
two being held in such relationship that the image of the 
sun formed on the screen is round. The screen is then to 
be moved back and forth until the point is found at which 
this image is smallest, and the distance of such point from 
the lens is the focal length of the lens. To learn the strength 
of the lens in diopters, we divide ioo centimeters (one 
meter) by the focal length expressed in centimeters, or 
40 inches (about one meter) by the focal length expressed 

63 



64 SPECTACLES AND EYEGLASSES. 

in inches. For instance, if we found the focus of the lens 
under examination to be distant 10 in., or 25 cm., from the 
lens, 40 in. divided by 10 in., or 100 cm. divided by 25 
cm., will alike give a quotient of 4, and the lens measured 
was, therefore, a + 4. D. 

The focal length of a concave lens may be similarly meas- 
ured by combining it with a stronger convex lens and then 
measuring the strength of the resulting weaker convex. 
The strength of the original convex used being known, we 
have only to subtract from it the weak convex resultant to 
find the strength of the concave with which we are dealing. 
The focal length of convex and concave cylindrical lenses 
may be measured in the same way as the corresponding 
sphericals, it being only necessary to observe that the 
parallel rays of light after passing through a convex cylin- 
drical lens are arranged in the form of a line at the focus of 
such lens; not brought to a point, as is the case with con- 
vex sphericals. 

Phacometers. — Such methods as the one described 
above are, however, too tedious for ordinary use, though 
quite elaborate contrivances called phacometers have been 
devised on this principle. A lens measure constructed on 
an entirely different idea has appeared, the invention 
of Mr. J. T. Brayton, of Chicago. Fig. 37 shows the size 
and appearance of the instrument, as well as the method of 
its use. Of the three steel pins which project from its top 
the two outer ones are fixed, while the central one moves 
up and down easily but is held up by a spring. On press- 
ing the surface of a spherical lens squarely against these 
points the central one will be depressed until they all 
three touch the glass, the curvature of the surface of the 
lens determining the amount of such depression. The 
motion being transferred through a rather simple mechan- 
ism to the hand upon the dial, this travels over a scale 
which shows in diopters the strength of the lens corre- 



INSPECTION AND ADJUSTMENT OF SPECTACLES. 



65 



sponding to the surface tested. The other surface is then 
to be explored in the same way. If the lens is bicon- 
vex or biconcave, the results of measuring each surface 
separately are added together; if periscopic, the less is de- 
ducted from the greater. When used upon a cylindrical 
surface the hand will stand at zero when the three points 
are in line with the axis of the cylinder. When the points 

Fig. 37. 




are placed at right angles to the axis the strength of the 
cylinder is shown. 

Since this instrument indicates the refractive value of a 
lens from the curvature of its surfaces only, leaving out of 
account the index of refraction of the material, it is evident 
that it can be accurate for only one variety of glass. As 
found in the shops it is adjusted for crown glass, and for 
lenses of this material it is quite accurate; while its con- 
venience and low price as compared with other phacometers 
recommend it to favor. 
6 



66 SPECTACLES AND EYEGLASSES. 

Neutralization of Spherical Lenses. — The method of 
determining the strength of spectacles which is of most gen- 
eral utility is the well-known one of neutralization. If a 
convex spherical lens be held about a foot from the eye, 
and any object, say that part of a window frame where a 
vertical and horizontal line cross, be viewed through it, any 
motion given the lens will result in an apparent motion in 
the opposite direction of the object sighted. That is, if the 
lens is moved to the right, the object appears to move to the 
left; if the lens is raised the object appears to sink. If the 
same maneuver be employed with a concave spherical glass, 
the object again appears to move, but this time in the same 
direction as the motion imparted to the lens ; if the lens is 
moved to the right, the object appears to move to the right 
also. Here we have the readiest possible means of distin- 
guishing between a convex and a concave lens. Moreover, 
one gets in this way an idea of the strength of a lens, as the 
stronger the lens the more rapid is the apparent motion of 
the object seen through it. 

If, continuing the experiment, the two lenses be placed 
together, with their curved surfaces in apposition, and a 
trial be made of the effect of moving them before an object, 
as was done previously with each lens singly, the object 
will appear : i (if the concave lens is the stronger) , to move 
in the same direction as the motion of the glass, but more 
slowly than before; 2 (if the convex lens is the stronger), 
to move in the opposite direction to the motion of the 
glass, but more slowly than before; 3 (if the lenses are of 
equal strength), to have no motion. Therefore, to find the 
strength of a spherical lens it is only necessary to combine 
it in this way with successive lenses of known strength and 
of the opposite sign until that one is found which neutral- 
izes the apparent motion of objects seen through it. This 
lens is the measure of the strength of the one tested. This 
method is accurate within an eighth diopter, or less, for 



INSPECTION AND ADJUSTMENT OF SPECTACLES. 



6 7 



plano-convex and plano-concave lenses; with bi-convex 
and bi-concave glasses it is only possible to neutralize the 
apparent motion near the center of the lens; toward the 
edges motion will still be visible when the lenses are strong. 
Cylindrical lenses may be recognized by viewing through 
them some object presenting a straight line, say the vertical 
line of a window sash. If the cylindrical lens be rotated 
about the visual axis, the portion of the vertical line seen 
through the glass will appear to be oblique, as compared 
with that seen above and below the glass (Fig. 38). This 
oblique displacement takes place in a direction contrary to 



Fig. 38. 



Fig. 39. 





the rotary motion given the lens if the latter is convex, and 
in the same direction as the motion if the lens is concave. 
To ascertain the position of the axis of a cylindrical lens it 
should be rotated slowly in this manner until the line seen 
through it appears continuous with that above and below 
(Fig. 39). This line will then lie either in the axis or at 
right angles to it. To ascertain which of the latter is the 
case, the effect of motion from side to side is to be tried. 
If the axis of the cylinder corresponds with the vertical line 
looked at, motion from side to side produces apparent 
motion of the object; if, however, the axis lies at right 



:S SPECTACLES ANI EYEGLASSES 

angles tc the vertical line no such motion results. In other 
words, in the direction of its axis a cylindrical lens acz as 
a piece of plain glass: across its axis it acts as a spherical 
lens :: the same strength. I: it is ilesired to know upon 
which surface of a lens the cylinder is ground, this may be 
ascertained by holding the lens nearly horizontally between 
the eye and a window, so that the line of sight strikes its 
upper surface very obliquely. One can thereby see the 
lines of the window reflected upon the upr-r surface of the 
lens. By rotating the lens about its optic axis these lines 
appear broken if the surface is cylindrical, but retain theii 
continuity if the refecting surface is spherical. The direc- 
tion of the axis :: a cylindrical lens having been ascertained, 
its strength may be determined by neutralizing it with a 
cylinder of the opposite sign, as was explained when speak- 
ing of spherical lenses. Care must be taken that the two 
lenses are s: placed that their axes coincide. 

A Sphero-cylindrical Lens is equal in refractive effect 
to two cylindrical lenses with their axes perpendicular a: 
each other. Having found that axis across which motion 
is least rapid, we may neutralize the motion with a sphe- 
rical lens and. holding these twc together, proceed to neu- 
tralize the motion acrzss the other axis just as if dealing 
with a simple cylinder. When our object is not to deter- 
mine the strength of an unknown lens, but to see if the 
Lenses of a pair of spectacles agree with the prescription 
previously written, we may. of course, shorten the above 
procedures by picking out from the test case the glass :r 
glasses which will neutralize the spectacles if the latter are 
of the proper strength, and observing whether the apparent 
motion of objects ceases when they are held together. 

Locating the Optical Center. — Ever}* glass before be- 
ing worn should be examined with regard to the position 
of the optical center :: each lens and the distance of these 
from each other, as inaccuracy in this important particular 



INSPECTION AND ADJUSTMENT OF SPECTACLES. 



6 9 



is not uncommon. Indeed, in the cheap spectacles which 
some persons unfortunately buy, proper centering is the 
exception. In grinding large numbers of lenses by machin- 
ery a certain number in each batch are, I believe, always 
found to be badly centered. These are not returned to the 
wheel or the furnace by the thrifty manufacturer, but are 
graded as second class, or if very bad indeed as third class, 
and with those which will not pass inspection in other 
particulars go to make up the trash sold by peddlers. 

A simple way to find the location of the optical center is 
to hold the lens about a foot above the corner of a rectan- 




Fig. 40. 

gular card lying on the table. The corner seen through 
the lens will only appear complete and continuous with the 
rest of the card when its tip is opposite the optical center. 
In Fig. 40, A represents a lens so held that its optical 
center is marked by the corner of the underlying card; b 
is a lens improperly held. The center first found may be 
marked with a speck of ink, the center of the other spec- 
tacle glass found in the same way, and the distance between 
them measured. If care is taken to hold the glass exactly 
level and the eye directly over it this method will give 
results accurate enough for most purposes. 



70 SPECTACLES AND EYEGLASSES. 

The Apex of a Prism may be determined by viewing 
through the glass fine lines crossed at right angles, holding 
the prism so that its edge and supposed apex just touches 
one line at the point of intersection. When the real apex 
of the prism coincides with the intersection of the lines, the 
appearance presented is that shown in Fig. 41 ; when, how- 
ever, the apex is to one side of the point of intersection, the 
line seen through the prism appears broken, as in Fig. 42. 
In this case the prism is to be rotated until the line appears 
continuous, when the point of intersection of the lines will 
mark the apex of the prism. 

The Strength of a Prism may be expressed in two ways; 
either in degrees of the refracting angle, which is the angle 





Fig. 41. Fig. 42. 

Method of Finding the Apex of a Prism. (After Maddox.) 

forming the edge and separating the two refracting sur- 
faces of the prism, or by means of some formula which 
denotes the power of the prism to turn a ray of light from 
its course. This power is usually expressed in degrees 
of the angle of deviation, which is the angle separating the 
course of a ray of light after having passed through the 
prism from that which it would have pursued had its course 
been unobstructed. The obvious advantage of the latter 
mode of expression, which gives directly the optical strength 
of the prism, over the former, which merely states the 
value of a physical angle from which the strength can 
be more or less accurately inferred, has called forth several 
suggestions for an improved method of numbering ophthal- 
mological prisms. Dr. Edward Jackson was the first to 



INSPECTION AND ADJUSTMENT OF SPECTACLES. 7 1 

point out that the prism had escaped attention when the 
numeration of our other glasses was reformed. He pro- 
posed that in harmony with the mode of stating the value 
of angles which is commonly accepted in other depart- 
ments of science, they be marked in degrees of their angles 
of deviation. With the idea of conforming their numera- 
tion to the dioptric system of numbering lenses, Mr. C, F. 
Prentice proposed to adopt as a unit that prism having 
the power necessary to produce one centimeter of devia- 
tion in the course of the ray after having passed through 
and the distance of one meter beyond the prism. Dr. 
S. M. Burnett proposes that this unit be called the prism 
diopter, and that the centimeter of deviation be measured 
upon a plane surface — that is, upon a tangent of the arc 
whose radius is one meter. 

Within practical limits the objections which have been 
raised to the prism diopter are few and of little moment, 
and it has great simplicity to recommend it. In a series 
of prisms so numbered, however, the higher prisms are not 
simple multiples of the lower ones. Twenty prisms of two 
P. D. each would not be equal to a prism of 40 P. D., but 
to a prism of 42 P. D. 

The centrad as a unit of measurement of prism power 
was suggested by Dr. W. S. Dennett. After mature con- 
sideration this unit has been formally recommended by the 
American Ophthalmological Society, and will doubtless in 
a few years entirely, as it has already to a great degree, 
displace the old system of numbering. 

The term radian denotes in mathematics a portion of the 
arc equal to the radius. The centradian is the one hun- 
dredth part of the radian. The centrad is such a prism as, 
held with one surface perpendicular to the incident ray, 
causes a deflection equal to a centradian. If the measure- 
ment be made at one meter, then, the radius and radian 
being each one meter long, the centradian will equal a 



72 



SPECTACLES AND EYEGLASSES. 



centimeter, measured on the arc, and the centrad is such a 
prism as will produce this amount of deflection. If the 
measurement be made at two meters — a very convenient 
distance — one centrad will produce a deflection of one 
hundredth of two meters, or two centimeters. 

It will be seen that the practical difference between a 
centrad and a prism diopter consists in this, that in the 



TABLE VI.— SHOWING THE EQUIVALENCE OF CENTRADS IN 
PRISM DIOPTERS AND IN DEGREES OF THE REFRACT- 
ING ANGLE (INDEX OF REFRACTION 1.54)- 



Centrads. 


Prism Diopters. 


Refracting Angle. 


I 


1 


i°.oo 


2 


2.0001 


2°. 1 2 


3 


3.0013 


3 °. 18 


4 


4.0028 


4°-23 


5 


5-°°45 


5°. 2 8 


6 


6.0063 


6°. 3 2 


7 


7.0115 


7°-35 


8 


8.0172 


8°. 3 8 


9 


9.0244 


9°-39 


10 


10.033 


io°-39 


11 


11.044 


"°-37 


12 


12.057 


i2°.34 


13 


i3-°74 


13°.2 9 


14 


14.092 


I 4 °.2 3 


15 


15,114 


i 5 °.i6 


16 


16.138 


i6°.o8 


17 


17.164 


i6°. 9 8 


18 


18.196 


i7°-85 


19 


19.230 


i8°.68 


20 


20.270 


i9°-45 


25 


25-55 


23°-43 


30 


3°-934 


2 6°.8i 


35 


3 6 -5° 


29°-72 


40 


42.28 


3 2°.i8 


45 


48.30 


34° 20 


50 


54-514 


35°-94 


60 


68.43 


38°-3i 


70 


84.22 


39°- 73 


80 


102.96 


4 0°.29 


90 


126.01 


4o°-49 


100 


155-75 


39°- 14 



INSPECTION AND ADJUSTMENT OF SPECTACLES. 73 

former the amount of deflection is measured on the arc, 
while in the latter it is measured on the tangent. For 
ophthalmological prisms, which are of necessity weak, the 
difference between centrads and prism diopters is so slight 
as to be of no moment. The numeration of prisms by 
centrads has the advantage that it is founded on a method 
of. stating the value of the angle which is used in other 
departments of physics. Its higher numbers in the scale 
are, moreover, simple multiples of the unit. 

Over the system of numbering prisms in degrees of the 
refracting angle the use of the centrad has all the advan- 
tages possessed by the modern numeration of spherical 
lenses over the old. Its use, moreover, involves no per- 
plexity in the mind of one who has become habituated to 
the former method, since, as shown in Table VI the differ- 
ence in value of one of the old and one of the new prisms 
of the same number is slight for the weaker, more used 
prisms. The one, however, represents a definite, fixed 
value; the other does not. 

As the surgeon has a choice of two essentially different 
methods of numbering, so, also, he has at his command 
several modes of determining the strength of unknown 
prisms, and may select that one which is simplest and 
involves least calculation for the numeration which he 
uses. The refracting angle may be readily found by 
means of Table III, introduced when speaking of the pris- 
matic equivalent of decentered lenses. The situation of 
the optical center is to be marked upon a spherical lens of 
convenient strength, and the prism to be tested super- 
imposed. By viewing the corner of a card through these 
two glasses, as was directed in describing the method of 
finding the optical center, this center will be found to have 
been carried toward the base of the prism. The position 
of this apparent optical center is to be likewise marked 
upon the spherical lens, and its distance from the true one 
7 



74 



SPECTACLES AND EYEGLASSES. 



measured. In the left-hand column of Table III find the 
strength of the lens used, and on a level with this across 
the page the distance in millimeters between the true and 
apparent optical centers. At the head of the column in 
which this measurement is found will stand the strength of 
the prism with which the lens was combined, this strength 
being expressed in degrees of the refracting angle. For 



Fig. 43- 




A, 



instance, if having combined an unknown prism with a + 
7. D. lens we find the apparent displacement of the optical 
center to be 4 mm., the table shows at a glance that the 
refracting angle of the prism tested had a value of 3 . 

The refracting angle may be directly measured by 
adapting the legs of a pair of compasses to the two re- 
fracting surfaces and then laying the compasses on an 



INSPECTION AND ADJUSTMENT OF SPECTACLES. 75 

ordinary protractor. Various other mechanical contriv- 
ances have been invented for effecting the same purpose, 
one of the best of which is represented in Fig. 43. It con- 
sists of a bed-plate A, upon the front of which is affixed a 
degree-circle G, and hinged to A at H is the upper plate B 
held up by the spring M, not plainly shown because it is 
under B. The upright face-plate C stands at right angles 
to B. On top of C is the degree-circle B. The index 
linger F with the lower part D D' is made of steel and 
pivoted at P to swing easily over any portion of the dial 
plate. In measuring a prism, the position of the index 
finger F will be governed by the difference of the thick- 
ness of the lens at the points D and D', and the degrees of 
the refracting angle of the prism will be indicated on the 
scale K by the pointer F. 

. The surgeon is, however, very little concerned with the 
refracting angles of prisms, except as they are the basis of 
the old system of numbering, which will doubtless soon be 
superseded by one in which the number of the prism shall 

Fig. 44. 




express in centrads the power which that prism possesses 
of causing deviation in a ray of light. One of the simplest 
and most convenient devices for measuring this power is 
that suggested by Dr. Maddox. It consists of a strip of 
cardboard suspended horizontally on the wall on a level 
with the eyes of the observer. The upper border of the 
card (Fig. 44) is marked from right to left with a scale of 
degrees, or rather tangents of degrees, proper to the distance 
at which the prism is to be held from the card. In Table 



76 



SPECTACLES AND EYEGLASSES. 



VII is given the distance from the right-hand border of the 
card of the mark for each degree of deviating angle. With 
the help of this table one may readily construct the scale, 
using column A if he elect to work at six feet, or column B 
if a two-meter range be preferred. 

To practice this method of prismetry, the glass to be 
tested is held at the proper distance from the card, its apex 
to the left, and its upper border just below the figures of the 
scale, as in Fig. 44. The observer's eye being placed behind 
the prism, the right vertical border of the card appears dis- 
placed toward the observer's left and points upward to the 
number expressing the strength of the prism in degrees of 
the angle of deviation. During this maneuver care must 
be taken that the prism is held at precisely the distance 
from the card for which the scale of the latter is arranged ; 
also that the apex of the prism points exactly to the left. 
This latter requirement may be secured by rotating the 



TABLE VII.* 



For Marking a Card in Tangents of Degrees at 6 Feet 


(Column A) ; or 2 Meters (Column B). 






A 


B 




A 


B 


i° 


1.25 in. 


3.49 cm. 


9° 


11. 4 in. 


31.29 cm. 


2° 


2-5 " 


6.98 " 


IO° 


12.6 " 


34-73 " 


3° 


3-7 " 


10.467 " 


ii° 


14.0 " 


38.16 " 


4° 


5-o " 


13-95 " 


12° 


15-3 " 


41.58 " 


5° 


6-3 " 


17-43 " 


13° 


16.6 " 


44.99 " 


6° 


7-57 " 


20.9 " 


14° 


17.9 " 


48.38 " 


7° 


8.84 " 


24-37 " 


i.S° 


19-3 " 


5I-76 " 


8° 


10.12 " 


27-83 " 


1 6° 


20.64 " 


55-13 " 



prism until the line of the bottom of the card appears un- 
broken, as at a, in Fig. 44. In adapting this method of 
prismetry to centrads or prism diopters, the scale at the top 
of the card should simply be laid off in centimeters, and 



*From Maddox: " The , Clinical Use of Prisms. 



INSPECTION AND ADJUSTMENT OF SPECTACLES. 77 

the prism be held at the distance of one meter. Each cen- 
timeter that the right border of the card is apparently 
moved to the left, on viewing it through the prism, will 
then represent one centrad, or one prism diopter. 

Scratches, Specks, Bubbles, Flaws, etc., in the glass 
will hardly escape detection if they are carefully looked for 
while the lens is held in different lights. Placing the glass 
against a dark background and allowing a bright light to 
fall obliquely upon it will perhaps bring them out as plainly 
as any other maneuver. 

Irregularity of the Surface may be discovered by re- 
flecting from that surface any object having regular outlines. 
The observer should stand facing a window, holding the 
lens against a dark background in his left hand, and pass a 
straight-edged piece of paper held in his right hand between 
his eyes and the lens. Two images of the paper will be re- 
flected from the lens — one formed by each surface. Any 
irregularity of these surfaces will make the images appear 
broken, or with wavy outlines. 

Fig. 45- 



Adjusting Spectacle Frames. — It requires some little 
practice to enable one to tell at a glance just where such 
an irregularly shaped object as a spectacle frame has been 
wrongly bent; having found the error, it is a more simple 
matter to correct it. For the latter purpose two small 
pliers are required. They should have narrow but strong 
jaws, round in one pair and square in the other. As found 
in the shops, the grasping surfaces of the jaws are generally 

tore 



7« 



SPECTACLES AND EYEGLASSES. 



roughened, but should be smoothed off with a file, lest 
they scar the gold when in use. A small, stout screw-driver 
with a point suited to the screws of spectacles will also be 
necessary. 

Fig. 46. 




Eye wires are generally of such light material as to take 
their shape from the contained glass, and are, therefore, not 
liable to become misshapen. Sometimes the long axis of an 
oval eye gets rotated within the eye wire (Fig. 45), so that 
it no longer stands squarely across the face. By loosening 

Fig. 47- 




the screw it can readily be re-adjusted. Abnormal crooked- 
ness about the bridge is best disclosed by placing a straight 
edge (indicated by the line S E in Figs. 45, 46, 48, 49 and 
50) in such a position as to enable one to compare the two 
sides of the frame. If the bridge is bent at its junction 



INSPECTION AND ADJUSTMENT OF SPECTACLES. 



79 



with the eye wire a rotation results, looking very much 
like that just mentioned, but dependent upon an entirely 
different fault (Fig. 46). It is readily corrected with pliers 
or ringers. 

The planes of the glasses may cross each other (Fig. 47), 
in consequence of a twist in almost any part of the bridge, 

Fig. 48. 




Fig. 49. 





Fig. 





OO SPECTACLES AND EYEGLASSES. 

though the trouble is, usually, that the angle of the bridge 
at A is not of the same size as its fellow of the opposite 
side. The bridge is inclined, as shown in the cut, more to 
one glass than to the other. It requires application to the 
patient's face to determine which is the proper inclination, 
and in order that the glasses may be equalized at this and 
not at the improper one. 

In Fig. 48 the bend is at the junction of the eye wire 
with the bridge, rendering corresponding angles of the two 
sides of the frame unequal. The diagram shows the 
change necessary to correct the trouble. A similar fault 
is shown in Fig. 49. This appears at first sight to be just 
like the last; it is, however, a neighboring angle of the 
bridge which needs equalizing with its fellow. 

In the frame represented in Fig. 50 the glasses lie in 
the same plane, but one of them is nearer the center of the 
bridge than the other, due to the fact that, of the angles 
of the bridge which can be seen by viewing the frame in 
this position, the two which lie on one side of the curved 
portion are too much open, while the two on the other side 
are too little so. Of course, the bridge may be misshapen 
in any portion of its extent, but the illustrations given are 
sufficient to show the sort of faults one may expect. 

Having rectified all want of symmetry in the ''front," 
the defects in the fit of the temples can best be corrected 
by trying the frames on the patient's face. If on doing so 
it is found that their temples cut into the temples of the 
wearer, instead of just touching the skin, as they should do, 
the trouble is obviously that the distance between the tem- 
ples is too small, and they must be bent out at the hinges, 
so as to throw them, when open, farther apart. This is 
done with the square-jawed pliers, seizing the wire close 
up to the hinge. When the opposite condition pertains, 
that is, when the distance between the temples is too great, 
leaving a space between each wire and the side of the 



INSPECTION AND ADJUSTMENT OF SPECTACLES. 51 

wearer's head, they require to be bent in. To do this, take 
the end of each side in turn in the square-jawed pliers, in 
such a way that the edge of one jaw shall be in contact with 
the temple as close to the hinge as possible and the latter 
be held rigidly open. The temple may then be pressed in 
with the fingers, and will bend at the point where it is 
pressed against the edge of the pliers. If the latter are 
rightly placed this does not make an angle in the wire form- 
ing the temple, but simply alters the angle already formed 
at a in Fig. 50, by the expansion of the end of the temple to 
help form the hinge. Care must be taken that one temple 
is not bent out more than the other, or, as is apt to be the 
case, become so during use. When this happens the 

Fig. 51. 




effect is quite different from what might be expected. The 
glass on the same side as the temple the more bent out 
will be brought closer to the eye, while its fellow will be 
carried farther forward and the bridge will ride obliquely 
across the nose. To remedy this it is only necessary to 
equalize the divergence of the temples. 

The curve of hook temples given them by the maker 
will rarely be found to fit comfortably behind the ear. As 
has been pointed out by Dr. Charles H. Thomas, the proper 
form for hook temples is a straight line from the hinge to 
the top of the ear, where a sharp curve should join this part 
of the temple to the easy curve which corresponds to the 
back of the ear (Fig. 47). Where the curve given the hook 



52 SPECTACLES AND EYEGLASSES. 

is too wide and is extended upon that part of the wire rest- 
ing against the patient's temple, as shown by the dotted 
line in Fig. 51, there is a constant tendency of the spec- 
tacles to slide forward. The wire, moreover, touches the 
back of the ear for a short distance only, where its pressure 
is further increased by the fact of the whole temple being 
put upon the stretch and acting as a spring. Especially at 
first should the frames not fit too tightly, as the skin is then 
more easily irritated by the wire than when it becomes 
accustomed to its presence. 

In persons whose ears stand out far from the head a 
certain ridge upon the cartilage of the ear is thrown into 
prominence. Since the curve of a hook temple is a regular 
one, it will rest upon this ridge and be very uncomfortable; 
indeed, it may cut through the skin and into the cartilage. 
Under such circumstances the portion of the wire which is 
behind the ear should be made to follow every depression 
and elevation of the surface with which it is in contact ; as 
it should in any case where the auricle is deformed or 
irregular in any way. 

If one lens stands higher upon the face than the other, 
so that the patient looks through the upper part of one 
glass and the lower part of the other, it will be found that 
the temple on the side which stands the higher is turned 
down more than its fellow. It should be raised, or more 
frequently its fellow should be lowered. The fault may 
lie in the bridge, as shown in Fig. 47, or in the end piece, 
or in the temple itself. In the first instance, bringing the 
lenses into the same plane removes the difficulty; in the 
second, take the end piece in the round-jawed pliers, the 
jaws being applied to its edges close up to the eye wire. 
Holding these pliers in the left hand, apply the square jaws 
of the other pliers to the surfaces of the end piece; when, 
by twisting the latter about its long axis, the temple may 
be turned down to any desired extent. Thus, the temple 



INSPECTION AND ADJUSTMENT OF SPECTACLES. 



83 



is not bent at all, but the end piece between the hinge and 
the eye wire. Nearly the same effect may be produced by 
bending the wire of the temple close up to the hinge. As 
was remarked before, in speaking of the facing of the glasses, 
the effect of turning down both temples is not to make both 
lenses stand higher upon the face but to make the glasses 
face more downward. 

Sometimes when the glasses do not sit properly the 
trouble will be found to be not in the frames but in the 
wearer. A considerable amount of asymmetry of the two 
sides of the face is not uncommon. One ear or one eye 

Fig. 52. 




may be higher than its fellow, either of which conditions 
will make the glasses seem awry, and render necessary a 
compensating asymmetry of their frames. 

Adjustment of Eyeglasses. — The starting-point in ad- 
justing eyeglasses is at the nose-pieces, whose free sur- 
faces should be made to conform accurately to the bones of 
the nose by which they are supported. When received 
from the maker they are generally curved, presenting a 
convexity toward the nose. As the bones of the sides of 
the nose at the point where the guards are to rest are 
usually more or less convex also, the bearing obtained is 
a most insecure and uncomfortable one, as a glance at Fig. 
52 will show. In Fig. 53 this glass is shown with its nose- 



84 SPECTACLES AND EYEGLASSES. 

pieces properly adapted to the sides of the nose. Any con- 
formation may be required, but that shown in Fig. 53 is 
the one most frequently needed. These changes in the 
shape of the nose-pieces are readily effected by means of 
the pliers, especially if the nose-pieces recommended in 
Part I are used. When the guards are of cork, care must 
be taken that they are not scarred and broken by the pliers, 
and a special tool with a longitudinal groove in the jaws 
for grasping the sides of the nose-pieces is here of service. 
It will be readily seen, moreover, that the nose-pieces 
must incline equally to a vertical plane passing through 

Fig. 53. 




the center of the nose; otherwise the glasses will stand 
awry; that is, provided the nose is straight and its two 
sides alike. In a large proportion of cases, however, a plane 
through the middle of the nose is only approximately ver- 
tical, so that one nose-piece must be inclined more than 
the other. (Fig. 54.) It is even necessary, sometimes, to 
incline the top of one nose-piece toward the vertical plane 
and the other away from it. 

The slope of one side of the nose from the crest backward 
is frequently steeper than that of the other side. A sym- 
metrical eyeglass on such a nose will stand with one lens 
close to the corresponding eye and the other standing for- 
ward away from the eye. One nose-piece will tend to 



INSPECTION AND ADJUSTMENT OF SPECTACLES. 85 

rest on its anterior edge and the other on its posterior edge. 
By partially revolving each nose-piece around the long axis 
of its bearing surface the glasses are brought parallel to the 
general plane of the face and each nose-piece presses evenly 
over its whole bearing surface. Fig. 55 is intended to il- 



Fig 




lustrate what is meant. It is a diagrammatic view from 
above, showing the ends of the nose-pieces as arranged for a 
case in which the bridge of the nose does not slope equally 
on the two sides. These changes in the inclination of the 
nose-pieces are brought about by bending or twisting the 
foot at the point b in Fig. 56. 

Fig. 55. 





Having conformed the nose-pieces to their bony support, 
the tension of the spring by which they are pressed against 
the sides of the nose is to be regulated, the object being to 
have just sufficient force exerted to keep the guards 
securely in place. If the latter are properly fitted the 



86 



SPECTACLES AND EYEGLASSES. 



amount of pressure necessary is not great. Though this 
pressure should be evenly distributed over the surfaces of 
the nose-pieces, want of firmness in the "pinch" of their 
tops is particularly fatal, as the lower ends then become 
the principal support of the weight of the glasses, render- 
ing them prone to topple forward and fall. To increase 
the tension of the spring, and consequently the pinch of 
the frames, the curve of the spring included between the 
lines at A, in Fig. 56, should be made more arched and 
rounded. Conversely, the force of the spring is lessened 

Fig. 56. 
A 

A 




by flattening this arch. Any alteration in the shape of the 
spring, however, while it does not, of course, change the 
shape of the nose-pieces, does change the angle at which 
they are inclined to each other. For instance, if the spring 
be made more arched, the nose-pieces are brought nearer 
together, but the bottoms are especially approached toward 
each other. When the spring is flattened the bottoms of 
the nose-pieces are thrown proportionately farther apart 
than the tops. It follows that with each adjustment of the 
tension of the spring the inclination of the nose-pieces must 



INSPECTION AND ADJUSTMENT OF SPECTACLES. QJ 

be rectified. This is easily accomplished by twisting the 
"foot" or support of the nose-piece at b in Fig. 56. 

When the points mentioned have been properly adjusted, 
the long axis of one or both glasses may fail to stand 
squarely across the face as it should do. The remedy lies 
in an appropriate bend of the spring at the point c (Fig. 56). 
This also requires a slight re-adjustment of the inclination 
of the nose-pieces to each other. 

The distance between the centers of eyeglasses is deter- 
mined (the distance between the nose-pieces when in use 
being a fixed quantity) by the distance of the nose-piece on 
each side from the center of the corresponding eye. The 
intercentral measurement may therefore be varied by vary- 
ing the size of the eye used, and by altering the distance of 
the nose-pieces from the edges of the lenses by an appro- 
priate bend of the foot b (Fig. 56), or by using a longer or 
shorter stud. The distance of the glasses from the eye 
is controlled by the length of the foot b, and in the better 
grades of goods this part is made in two or three lengths. 

Eyeglasses seldom stand too low upon the face but they 
frequently have the fault of standing too high, especially 
for near work. The neatest way of lowering them, but one 
which must be attended to when prescribing them, is to 
have the studs attached above the horizontal diameter of 
the lens, instead of at that diameter, as is usual. They 
may thus be lowered one, two or three millimeters; or 
a special form of nose-piece may be used. (Fig. 16.) It 
is sometimes necessary to combine these methods. 

The Care of Spectacles. — Spectacle frames will last 
longer and perform their function better if the wearer is 
instructed to exercise care in handling them. In putting 
them on and off, the hooks should be lifted from or into 
their position behind the ears; both hands being used, so 
as to avoid straining the temples widely apart or otherwise 
bending them. They should be folded together as little lis 



00 SPECTACLES AND EYEGLASSES. 

possible, and when not in use should be laid in a safe place, 
open, and resting on the edge of the lenses, to avoid scratch- 
ing the surfaces of the latter. For cleansing them nothing 
is better than a piece of clean old linen, or, if very much 
soiled, a little ammonia and water may be used, except 
on cemented bifocal glasses. While cleansing, the frame 
should be grasped by the end piece and not by the bridge, 
and in replacing the glasses on the eyes care should be taken 
not to crush them against the lashes and thus soil the re- 
fracting surfaces at once. When cylindrical or prismatic 
glasses are worn, patients may return after a time with the 
statement that the spectacles are unsatisfactory, when the 
trouble will frequently be found to be due to bending of the 
frame; or a lens may have fallen out and been replaced 
upside down, or with the wrong edge inward. It is well 
to have such persons report periodically to have their 
glasses re-adjusted. 



INDEX 



Adjustment of eyeglasses, 83 

of spectacles, 77 
Airy, discoverer of astigmatism, 7 
Alhazen, 3 
Ancient glass, 1 
Angle, deviating, of a prism, 71, 75 

refracting, 70, 73 
Angle of crest of bridge, 59 
Apex of a prism, finding the, 70 
Assyrians, knowledge of lenses 

among, 2 
Astigmatism, discovery of, 6 
Asymmetry of the face, 83 

Bar spring eyeglasses, 32 
Bifocal glasses, 26 

invention of, 7 
varieties of, 26 
Brewster, Sir David, 2, 6 
Bridge, width of base of, 59 

relation of top of to plane of 

glasses, 44, 58 
height of, 57 
Bridges, manufacture of, 21 
varieties of, 23 

Care of spectacles, 88 
Cemented bifocals, 27 
Centering and decentering, 36 

normal lateral, 41 

normal vertical, 43 

of spectacles for constant use, 42 
near work, 42, 44 
Centrad, 39, 71, 76 
Chemical composition of glass, 12 
Component parts of spectacles, n 
Conformation of nose-pieces, 83 
Crest measure, 60 
Crown glass, 12 
Cylinder, finding the axis of, 67 

Date of invention of spectacles, 4 
Decentered lenses, 36 

prismatic effect of, 38 

8 89 



Deviating angle of a prism, 71, 75 
Dioptric system, 17 
Di Spina, Alessandro, 5 
Discovery of astigmatism, 6 
Distance between the pupils, 36, 51 

between temples, 80 

of the glasses from the eyes, 44 

Emerald used by Nero, 2 
Epicanthus, eyeglasses for, 35 
Eyeglasses, advantages and disadvan- 
tages of, 9 

inspection and adjustment of, 
62, 83 

prescription of, 62 

varieties of, 30 
Extra front, 30 
Eye wires, manufacture of, 21 

Eace, asymmetry of, 57, 60, 83 
Facing of spectacles, 47 
Focal length of lenses, 63 
Frames (see spectacle frames) 
Frameless spectacles, 22 
Franklin, inventor of bifocal glasses, 

7 
glasses, 26 
Fused bifocals, 28 

Geometrical center, 36 

Glass, chemical composition of, 12 

ancient, 1 
Ground bifocals, 28 

Height of bridge, 57 
Hook temples, 23, 81 

Index of refraction, 13 
Inspection and adjustment of spec- 
tacles, 63 
Interpupillary distance, 36, 47, 51 
Introduction, 1 
Invention of spectacles, 4 
Irregularity of surface of lenses, 77 



9° 



INDEX. 



Kepler, Johann, 3 

Lateral centering, 41 
Lathe for grinding lenses, 17 
Lens cutter, 19 

oldest known, 2 
Lenses, decentered, 36 

finding focal length of, 63 

forms of, 13 

known to the ancients, 2 

material of, 12 

method of grinding, 17 

neutralization of spherical, 66 
cylindrical, 67 
sphero-cylindrical, 68 

numeration of, 17 

proving the strength of, 63 

toric, 20, 49 

tilted, 45 
Lenticular segments, 27 
Locating the optical center, 68 
Lorgnettes, 9 

Maddox pupil localizer, 55 
Marking of gold spectacle frames, 

10 
Material of lenses, 12 

of spectacle frames, 9 
Meniscus, 14 

Natural position for spectacle bridge, 

Nero, concave jewel used by, 2 
Neutralization of spherical lenses, 66 
cylindrical lenses, 67 
sphero-cylindrical lenses, 68 
New system of numbering lenses, 

Normal position of spectacles, 36 
Nose-pieces, conformation of, 8^ 
Numeration of lenses. 17 
of prisms, 70 

Offset guard, ^ 

Oldest known lens, 2 

Old system of numbering lenses, 17 

Opticians' lathe, 17 

Optical center, 36 

locating the. 68 

Pebble spectacles, 12 
Periscopic glasses, 20, 48 



Pince nez (see eyeglasses) 

Phacometers, 64 

Plane of the glasses, relation to the 
visual axis, 45 

Prism diopter, 71 

finding the apex of, 70 
deviating angle of, 70, 75 
refracting angle of, 70, 74 
numeration of, 70 

Prismatic effect of decentering, 38 

Prismetry, 73 

Prescription blank for spectacles, 53 
of eyeglasses, 62 
of frame, 51 

Proving the strength of lenses, 6^ 

Principal axis, 36 

Principles of spectacle fitting, 36 

Pupil localizer, 55 

Pupillometer, 56 

Quizzing glasses, 9 

Refracting angle of a prism, 70, 74 

surfaces, 13 
Rigid frame eyeglasses, 32 
Rock crystal, 1, 12 
Romans, knowledge of lenses among, 

2 
Rule for measuring frames, 53 

Saddle bridge, 23 

Salvinus Armatus, 4 

Scratches, specks, flaws, etc., in 

glass, 77 
Segments, lenticular, 27 
Signs used in prescription writing, 16 
Spectacle eyes, sizes of, 24 
shapes of, 24 

fitting, principles of, ^6 

frames, adjustment of, 77 

material of, 9 

marking of gold, 10 

prescription of, 51 

rule for measuring, 53 
Spectacles, component parts of, 11 

bifocal, 26 
Spectacles, care of, 88 

date of invention of, 4 

early references to, 4, 5 

for cosmetic effect, 35 

invention of bifocal, 7 

inspection and adjustment of, 6^ 



INDEX. 



Spectacles, frameless, 22 

facing of, 47 

patterns of, 22 

pebble, 12 

periscopic, 20, 48 

reversible, 24 
St. Jerome's eyeglasses, 5 
Surface, irregularity of, 77 

Temples, distance between, 80 
manufacture of, 21 
varieties of, 23 

Tilted lenses, 45 



" Tool" for grinding spherical lenses, 

17 
Tools for adjusting frames, 77 
Toric lenses, 20, 49 
Transparent glass found in Nineveh, 

1 
Trial frames, 51, 62 

Vertical centering, 43 
Visual axis, relation of plane of the 
glasses to, 45 

Width of base of bridge, 59 



MAH 30 1908 



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